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Galls dLivd Insects Producing Them 



By 



Melville Thurston Cook, A. M. 



r ■ ■ '-* • • 9 



Presented to the Faculty of the College of Arts, Philosophy 
and Science, Ohio State University, as the thesis requirement 
for the degree of Doctor of Philosophy. June 1904. 



• Cs 



MAR 



Contents 

Part I. The Morphology of Leaf Galls. 

Part II. Apical Bud Galls. 

Part III. Lateral Bud Galls. 

Part IV. Stem Galls. 

Part V. Development of Galls. 

Part VI. Flower and Fruit Galls. 

Part VII. Root Galls. 

Part VIII. Histology of Galls. 

Part IX. The Ovipositors and Mouth Parts of Gall Producing 

Insects. 
Appendix. 



P^K^ 



Parts I and II published in Ohio Naturalist Vol. II. pp 
263-278. 1902 

Parts III. IV. V. published in Ohio Naturalist Vol. 
III. pp 419-436. 1903. 

Parts VI. VII. VIII. IX and appendix published in 
Ohio Naturalist Vol. IV pp 115-147. 1904. 



[Rkprinted from Ohio Naturalist, Vol. II, pp. 263-27S, 1902.] 



GALLS AND INSECTS PRODUCING THEM. 

Melville Thurston Cook. 
Part i. The Morphology of Leap Gales. 

The purpose of this study was to contribute to the knowledge 
of cellular activity of the plant under peculiar animal stimulus ; 
to compare the effects of the two sets of insect organs, mouth 
parts and ovipositors, and to throw additional light on the classi- 
fication. The statements made in this paper are based on a large 
number of collections. The collection of stem galls was too 
incomplete to draw conclusions and is therefore reserved for a 
future paper. No attempt was made to follow the development 
of the galls but rather to make a comparison of the structure of 
the various forms of galls. 

My paper was practically complete before I received the papers 
of H. Fockeu. After receiving his paper I reviewed my own to 
determine wherein my results agreed with or varied from his 
conclusions. Experiments such as are described by H. Fockeu 
to ascertain the cause of the gall formation were not attempted. 

Fockeu' s studies were grouped according to the plants affected ; 
my own studies were grouped with reference to the insect pro- 
ducing the galls. 

METHODS. 

For the killing and fixing, several fluids were used, but the 
most successful were Chromo-acetic and Picric-alcohol. A num- 
ber of different stains were used, but Delafields-Haemotoxylon 
proved very satisfactory for most work. 

For the drawings a Bausch & Lomb microscope and camera 
lucida were used ; for the normal leaf, a i-inch ocular and a 
^-inch objective, and for the galls a i-inch ocular and a ^3-inch 
objective. Since it was unnecessary to make drawings of the 
entire galls, drawings were made from one or more parts to show 
the characteristic structure, and this part is indicated on the small 
diagrammatic drawings. Since the galls were so variable in size, 
it was practically impossible to make the diagrammatic drawings 
on a definite scale. 

GENERAL CLASSIFICATION. 

As a matter of convenience the following temporary classifica- 
tion, based on location of the galls was adopted for this and other 



264 The Ohio Naturalist. [Vol. II, No. 7, 

papers now in preparation: A. Stem galls; B. Leaf galls; 
C. Bud galls, a. Terminal buds, b. Lateral buds ; D. Root galls. 

Leaf galls may in many cases be classed as bud galls if we con- 
sider that the egg in some orders of insects is deposited while the 
leaf is in the bud, but in the above classification the term applies 
to the developed gall, and the ' bud gall ' applies to a distortion 
of the entire bud. 

i. The Normal Leaf Structure and Its Variations. 
The normal leaf structure may be said to consist of a single layer 
of epidermis on the upper and lower surfaces of the leaf ; next to 
the upper epidermis is the usually single layer of palisade or 
columnar cells, placed with their long axis at right angles to the 
surface of the leaf ; between the palisade cells and the lower epi- 
dermis is the mesophyll, made up of many layers of irregular 
cells, between which are the large air spaces connected with the 
outside by the stomata in the lower epidermis ; running through 
the leaf are the fibro-vascular bundles noticable to the naked eye 
as the venation. 

Although the above may be said to be a description of a typical 
leaf, it must be kept in mind that leaves are subject to great 
variation and this must be taken into consideration in a discus- 
sion of the variation of the gall structure from the normal leaf. 
The structure of the gall must be compared with the structure 
of the normal leaf of the plant on which the gall is found, not 
with the typical leaf. 

A brief study of the normal leaves of the plant will serve to 
emphasize the preceding points. Hicoria ovata ( Mill. ) Britton 
(Fig. i), Uhiuis americana L. (Fig. 4), and Tilia americana L. 
(Fig. 6) may be considered as typical and yet in themselves show 
minor differences. In Vitis vulpina L. (Fig. 3) the palisade is 
not so pronounced as in the preceding and the mesophyll is more 
compact. In Quercus alba L. (Fig. 7) and in Acer saccharin um 
L. (Fig. 5) the palisade is typical, but the mesophyll is very 
compact. In Sah'x cordata Muhl. (Fig. 2) the mesophyll while 
distinct from the palisade has assumed palisade characters. 

The differences in structure between the normal leaves of 
Hicoria ovata (Fig. 1) and Salix cordata (Fig. 2), members of two 
related families, are as great as those differences frequently found 
between a normal leaf and the galls occurring upon it, e. g., H. 
ovata (Fig. 1) and the simpler Phylloxera galls (Figs. 16-20). 

2. Phytoptus Galls. This discussion is based not only on 
the four galls described below, but from observations of several 
others. However, the following will illustrate all the points 
observed : 

The Phytoptus galls are small and may extend on either or 
both sides of the leaf. The outer surface of the galls show the 
normal epidermis and below this cells which are not palisade but 



May, 1902.] Galls and Insects Producing Them. 265 

which are elongated with the surface of the gall, i. <?., the direc- 
tion of growth (Figs. 8, 9, 11). Projecting into the gall cavity 
are masses of irregular shaped cells (Figs. 8-1 1). In young galls 
these cells show a nucleus, take the stain readily and show 
indications of maturity (Figs. 9, 11). Trichomes are always 
found extending from the walls of the cavity (Figs. 8-1 1) of 
young galls, but disappear as the galls approach maturity. In 
these galls we evidently have a repeated puncturing of cells by 
the animal and an increased activity on the part of the plant in 
its effort to recover from the wound, the wound never being suf- 
ficient to cause the death of that part of the plant. 

My results on the Phytoptus galls agree with those of H. 
Fockeu, except in minor points. 

3. The Aphididae Gaels. In this family we find the 
simplest form of galls discussed in this paper, of which Schizonc- 
ura americana Riley (Fig. 12) may be taken as a type. In fact 
it is a mere curling of the leaf and not what is usually considered 
a gall. According to E. Perris it would be classed as a galloide. 
However, the structure is very similar to that of a typical gall of 
this family of insects and I see no reason why it should not be 
considered a true gall. 

When compared with the normal leaf of U. americana L. 
(Fig. 4) the palisade cells are observed to have lost their identity 1 
and to have assumed mesophyll characters and the mesophyll has | 
become more compact, both distortions being characteristic of 
true galls of this family ( Figs. 13-21 ). 

In Coloplia ulmicola Fitch (Fig. 13 a. b. ) and Pemphigus ulmi- 
fusus (Walsh.) Oestlund (Fig. 14 a. b.) both of which are also 
characteristic galls on the elm, we find practically the same 
structure as in S. americana. In both the outer (upper) epi- 
dermis is much elongated ; the same being true of the inner 
(lower) epidermis of C. ulmicola, but not in P. ulmi-fusus. The 
identity of the palisade cells is entirely lost, the cells now being 
slightly elongated parallel to the surface of the gall. The 
mesophyll cells are more compact than in S. americana and far 
more compact than in a normal leaf (Fig. 4). 

A granular, dark brown, often black substance in the cells was 
characteristic of the elm and other galls of this group. This was 
probably tannin, and its presence seemed to depend on the host 
plant rather than on an insect producing the gall. 

The Hormaphis hamamelis Fitch ( Fig. 15 a. b. ) on the 
Hamamelis virginiana L,. showed the same general structure as 
the preceding galls of this order, except that the epidermal cells 
were not so much elongated and in the inner (lower) epidermis 
the cells were much smaller and showed thicker walls, and the 
dark granular contents of certain cells was restricted to layers 
near the outer (upper) surface. 



266 The Ohio Naturalist. [Vol. II, No. 7 r 

The Phylloxera galls show considerable variation from each 
other. P. c. avenae Fitch, P. c. fallax Riley, and P. c. globuli 
Walsh. (Figs. 16-18), of Hicoria ovata may be taken as forming 
a rather well defined group and as showing greatest resemblance 
to the preceding galls of this family. When compared with the 
normal leaf (Fig. 1) of the host, H. ovata, they show a reduc- 
tion in size of the epidermal cells, the palisade cells losing their 
identity, and the mesophyll becoming very compact. Very little 
of the dark cell contents characteristic of the preceding galls of 
this family was present, the greatest amount being formed in 
P. c. avenae (Fig. 16 ) where it is restricted to the epidermis and 
to the cells just below it. The cells are even less elongated and 
more irregular than in the preceding galls. In general it may be 
said that in this group the largest cells are midway between the 
two layers of the epidermis and gradually decrease as we approach 
the surfaces. This is especially true of P. c. globuli (Fig. 18 ). 

P. c. spinosa Shinier (Fig. 19 a. b. ) is a very large gall occur- 
ring on leaf, petiole, or young, green twigs of Hicoria ovata and 
shows considerable variation from the preceding. Two zones are 
very distinct ; the outer is composed of large cells which do not 
take the stain readily, the inner zone of small cells stained very 
readity and show great activity. This may, however, have been 
due to the fact that my specimens of this gall were much younger 
than of the preceding Phylloxera galls. A long tube for the exit 
of the insect is formed. 

In P. c. depressa Shinier (Fig. 20 a. b. ) of H. ovata and P. 
vastatrix Planchon (Fig. 21 a. b. ) of Vitis vulpina we have still 
other and more marked variation. The cavity is much. smaller, 
the walls much thicker than in the preceding, and a long tube, 
especially in P. c. depressa is formed for the exit of the insect. 
In both cases the size of the epidermal cells is much reduced when, 
compared with the normal (Fig. 1, 3), the palisade cells have not 
so completely lost their identity as in the preceding and there 
appears to be a general elongation of the cells with their long 
axis perpendicular and not parallel to the surface of the gall. 
A small but definite, deeply staining zone of cells surrounds the 
cavity in P. c. depressa. Many cells show dark contents similar 
to that found in the galls on Ulmus and Hamamelis (Fig. 12-15). 

P. vastatrix shows a comparatively large number of trichomes, 
especially near the opening, but this is probably a characteristic 
of the host plant rather than of the gall. 

The presence of the two well defined zones, which may be con- 
sidered protective and nutritive in P. c. spinosa and P. c. de- 
pressa, show a very marked resemblance to the Cynipidae galls 
(Figs. 25-30). 

It may be that all young galls show this arrangement into two- 
or three zones. 



May, 1902.] Galls and Insects Producing Them. 267 

In P. c. depressa (Fig. 20) and in P. vastatrix (Fig. 21) the 
small larval chamber and general arrangement of the cells is very 
similar to the leaf galls produced by Cecidomyia verrucola (Fig. 2.) 

4. The Cecidomyia Gales. This group of galls shows con- 
siderable variation. C. gleditsiae O. S. (Fig. 22 a. b. c. d. ) of 
Gleditschia triacanthos may be taken as a type of one of the 
simplest. In this the margins of the leaflets are in contact so as 
to form a more or less sperical body. To the naked eye it pre- 
sents no other distortion. Under the microscope the cells show 
an elongation from midrib to margin, i. e., parallel to the surface 
of the gall except near the margin, where they are irregular. 

C. quercus-pilulae Walsh. (Fig. 23 a. b. ) shows a more highly 
developed gall structure. The epidermal layers are made up of 
smaller cells than the normal leaf. The mesophyll has lost its 
identity and assumed the palisade structure, the long axis being 
perpendicular to the surface of the gall. The larval chamber is 
large and rather irregular and indefinite, and resembles a large 
inter-cellular space. 

C. verrucola O. S. (Fig. 24 a. b. ) on Tilia americana shows a 
much higher complexity than either of the preceding. The epi- 
dermis is made up of small cubical cells. The differentiation into 
palisade and mesophyll is entirely lost, the cells are very irregu- 
lar, but show a tendency to elongation at right angles to the 
surface of the gall. The larval chamber is small and well defined. 

C. q.-pilulae (Fig. 23) and C. verrucola (Fig. 24), especially 
the latter show a striking resemblance to the more highly devel- 
oped Phylloxera galls such as P. c. -depressa (Fig. 20) and P. 
vastatrix (Fig. 21). 

5. The Cynipidae Galls. This family presents the most 
striking series of evolutionary development of any family studied 
and is also apparently the most highly developed. 

The general characters presented by these galls are small, 
cubical epidermal cells ; loss of differentiation between palisade 
and mesophyll cells, all having assumed an irregular character ; 
a differentiation into two well defined zones of cells, the outer 
made up of large, non-staining cells, the inner made up of smaller, 
deeply staining cells and surrounding the larval chamber. 

Fockeu divides these into four zones, which he designates as 
follows: 1. Epidermis; 2. Parenchyma; 3. Protective; 4. 
Nutritive ("Masse alimentaire "). These four zones may be 
easily traced in most of our American forms, but in some they 
show very indistinctly. 

Neuroterus irregularis O. S. (Fig. 25 a. b. ) is a small, fleshy, 
solid, irregular gall projecting from both sides of the leaf. It is 
covered with dense growth of trichomes and contains several 
larval chambers. In structure it does not correspond to the pre- 
ceding description, as well as the galls described in the latter part 



268 The Ohio Naturalist. [Vol. II, No. 7, 

of this paper. The parenchyma is divided into two very distinct 
zones, the larval chamber occupying the lower part of the inner 
zone. The inner zone cells have much thinner walls than those 
of the outer cells. Surrounding the larval chamber is a zone of 
cells which stain very deeply and probably furnish nourishment 
to the larva. The epidermal cells are small. 

Callirhytis tumifica O. S. (Fig. 26 a. b.) is a small, fleshy, solid 
gall projecting on both sides of the leaf and resembles N. irregu- 
laris (Fig. 25), except that it is a little larger, does not have so 
many larval chambers and is smooth. It presents the simplest 
characters studied, showing the characteristic small, more or less 
cubical epithelial cells, the lack of differentiation into palisade 
and mesophyll, and the two zones. The outer zone is very thick 
and is in contact with the inner zone. The inner zone is narrow 
and lies near the large larval chamber. At the point of union of 
the two zones the cells are very small. The outer zone can be 
readily subdivided into epidermis and parenchyma, but the inner 
zone cannot be subdivided into two sub-zones unless we consider 
the layer of small cells as the protective sub-zone. However, 
this sub-zone of small cells does not possess the sclerenchyma 
character described by Fockeu for the Cynipidae galls. 

Holcaspis centricola O. S. (Fig. 27 a. b. c. ) is a large, spherical 
gall projecting both above and below the leaf. In this we have 
the two zones, but each retaining the characters previously 
described ; the cells of the inner zone, however, being smaller 
than in C. tumifica. The epidermal cells have thicker walls than 
in any other Cynipidae gall examined. The two zones are con- 
nected by fibro-vascular bundles. In this the four zones of 
Fockeu are quite well defined : The outer zone forming the very 
distinct epidermis and parenchyma ; the inner zone showing a 
fairly well defined protective and nutritive part. 

Amphibolips inanis O. S. (Fig. 28 a. b.) shows a very striking 
resemblance to H. centricola (Fig. 27), except that it is much 
larger. The epidermal cells do not have such thick walls as in 
H. centricola and are much longer and narrower. The inner zone 
is readily subdivided into the protective and nutritive sub-zones 
described by Fockeu. The inner or nutritive sub-zone is made 
up of thin-walled cells with prominent nuclei, the outer or pro- 
tective sub-zone of sclerenchyma cells. The connection between 
the two main zones is by means of fibro-vascular bundles, the 
same as in H. centricola. 

Dryophanta pahistris O. S. (Fig. 29 a. b. c.) presents a condi- 
tion very similar to the two preceding galls, H. centricola (Fig. 
27) and A. inanis (Fig. 28), except that the fibro-vascular bundle 
connection between the two zones is not present ; the inner zone 
containing the larva forms a sphere which is free in the large 
chamber formed by the outer zone. 



May, 1902.] Galls and Insects Producing Them. 269 

The inner zone shows a marked resemblance to H. eentricola 
(Fig. 27). The subdivision into protective and nutritive parts in 
my specimens was not like the characteristic zones described by 
Fockeu ; the inner cells were apparently much thicker walled and 
more indefinite. However, I believe that younger galls would 
have shown the typical characters. The outer zone is thicker 
than in either H. eentricola (Fig. 27 ) or A. inanis (Fig. 28), but 
not so thick as in C. tumifica (Fig. 26). It can be readily sub- 
divided into epidermis and parenchyma and it also shows a fairly 
well defined endodermis, and in that respect differs from either 
H. eentricola or A. inanis. 

Callirhytis papillatus O. S. (Fig. 30 a. b. c), which is similar 
to the preceding Cynipidae galls, but shows considerable varia- 
tion from them. It is smaller than any of the preceding and is 
embedded in the leaf very similar to C. tumifica (Fig. 26). The 
two zones are separated, the outer being similar to A. inanis 
(Fig. 28), the inner zone surrounding two or three larval cham- 
bers instead of one. Next to the larva the cells are very large 
and thin and may be considered nutritive ; outside these we have 
well defined parenchyma or protective cells, and outside these we 
have two or three layers of cells well filled with protoplasm. The 
connection between the outer and inner zones is by single elon- 
gated cells, which are very rich in protoplasm. 

The evolutionary development of the preceding Cynipidae galls 
is evident. All show the two well defined zones, the outer non- 
staining made up of epidermis and parenchyma and the inner 
which takes the stain readily and is made up of two subdivisions, 
protective (or sclerenchyma cells) and nutritive (or parenchyma 
cells). In C. tumifica (Fig. 26) we have the two zones in con- 
tact ; in H. eentricola (Fig. 27) and in A. inanis (Fig. 28) we 
have a separation of the two zones which are now connected by 
fibro- vascular bundles ; in C. papillatus (Fig. 30) the two zones 
are connected by long, undivided cells ; in D. palustris (Fig. 29 ) 
we have a complete separation of the two zones. 

With the exception of N. irregularis (Fig. 25) and C. tumifica 
( Fig. 26) they all show a division into four zones as described by 
Fockeu. However, Fockeu does not describe a separation 
between the parenchyma and protective zones which is so charac- 
teristic of some of our American galls. I am inclined to consider 
our American Cynipidae galls as having reached a higher stage 
of development than the European forms. 

The larva in all species evidently draws its nourishment directly 
from the inner zone. In H. eentricola (Fig. 27) and A. inanis 
( Fig. 28 ) the inner zone evidently gets its nourishment through 
the fibro- vascular bundles ; in C. papillatus (Fig. 30) the supply 
of nourishment comes through the long filamentous cells ; in 
D. palustris (Fig. 29) it is probable that the larva is far advanced 



270 The Ohio Naturalist. [Vol. II, No. 7, 

in its development before the separation of the two zones and the 
nourishment remaining in the inner zone at the time of the separ- 
ation is sufficient to complete its development. 

Adler and Stratton after describing similar modifications in the 
European Cynipidae galls, say: "Besides these histological 
differences, the outward characters are also of varying complex- 
ity ; each infinitesimal improvement, which has been of service 
as a protection against parasites, or has been successful in secur- 
ing natural conditions favorable to the life and growth of the 
larva, has been preserved, and has formed the starting point of 
further beneficial variation. It is always that larva which has 
been able to induce successful morphological abnormalition, which 
is reproduced to continue the race ; the unsuccessful perish. The 
ruling force is natural selection ; it is impossible that intelligence 
or memory can be of any use in guiding the Cynipidae ; no 
Cynips ever sees its young, and none ever pricks a bud the sec- 
ond season, or lives to know the results that follow the act. 
Natural selection alone has preserved an impulse which is released 
by seasonally recurring feelings, sights, or smells,* and by the 
simultaneus ripening of the eggs within the fly. These set the 
whole physiological apparatus in motion, and secure the insertion 
of eggs at the right time and in the right place. The number of 
eggs is instinctively proportionate to the space suitable for ovipo- 
sition, to the size of the fully grown galls, and to the food sup- 
plies available for their nutrition." 

conclusions. 

1. Galls may be classified into two general groups, viz., those 
produced by mouth parts and those produced by oviposition. 
Those produced by oviposition may be considered the more highly 
developed. 

2. The family Cynipidae shows by far the highest develop- 
ment of gall structures. 

3. The morphological character of the gall depends upon the 
genus of the insect producing it rather than upon the plant on 
which it is produced ; i. e. , galls produced by insects of a partic- 
ular genus show great similarity of structure even though on 
plants widely separated ; while galls on a particular genus of 
plants and produced by insects of different genera show great 
differences. This seems to indicate that the stimulus of a partic- 
ular genus of insect is given to a particular part of the host plant 
or is of a peculiar kind, characteristic of that genus. However, 
if the stimulus of two different genera of insects be applied to the 
same part of the plant the results may be similar. (See Part II. ) 

4. Within each family we find certain morphological resem- 
blances ; e. g. , Aphididae. 

* Weisuiann, Essays on Heredity, Vol. I, p. 95. 



Plate iS. 




Cook on Gaixs. 



272 The Ohio Naturalist [Vol. II, No. 7 r 

5. The families show parallel lines of development from a low 
form of gall structure up to a high form . e.g., Aphididae and 
Cynipidae. 

6. I am inclined to believe that the modification of the plant 
tissue is purely mechanical. The loss of differentiation between 
palisade and mesophyll and the closing up of the intercellular 
spaces would be a natural result of rapid cell division. The 
elongation of cells in certain directions would be a natural result 
of mechanical tension arising from rapid growth. In the family 
Aphididae where the gall is primarily a folding of the leaf the 
elongation of the cells is parallel with the surface of the gall. In 
those galls where the formation is a thickening of the leaf the 
long axis of the cells is perpendicular to surface of the formation. 

7. The presence of at least two zones, of which the inner may 
be considered nutritive, is very common. 

8. The formation of the gall is probably an effort on the part 
of the plant to protect itself from an injury which is not sufficient 
to cause death. Both Adler and Fockeu consider that after the 
first stages of formation the gall becomes an independent organ- 
ism growing upon the host plant. 

9. Trichomes are far more prominent in galls produced by 
mouth parts than in those produced by oviposition. 

10. It appears from these studies that the histological charac- 
ters of the gall will prove very important in determining the 
characters of the species. 

Part II. Apical Bud Galls. 

In my third conclusion in the preceding paper I have expressed 
a belief that galls produced by the same genus of insects show a 
decided resemblance even though produced on widely different 
plants. Furthermore, this similarity seemed to be due to the par- 
ticular part of the host plant to which the stimulus was applied. 

The following study of the apical bud galls seem to indicate 
that when corresponding parts of different plants are stimulated 
by insects of different genera that the galls produced have char- 
acters in common. 

The gall produced by Cecidomyia solidaginis J^w . (Fig. 31) is 
merely a large bunch of leaves at the end of the stem of Solidago. 
The cone-shaped gall of Cecidomyia salicis-strobiloides O . S. (Fig. 32) 
at the tip of the twigs of Salix is a bunch of leaves reduced in 
size and so compactly arranged as to produce the peculiar cone 
effect. A further examination of these two galls shows that the 
tips of the stems are enlarged and that the larval chamber is in 
the apex. 

A superficial examination of the gall of Callirhytis claxnda Fitch 
(Fig- 33 a - b. c - d.) shows no resemblance to the preceding galls 
except in location at the tip of the stem. The gall is apparently 



Ohio Naturalist. 



Plate ig. 




:MU ^ : ^M : 



Cook on Galls. 



274 The Ohio Naturalist. [Vol. II, No. 7, 

a mere enlargement of the tip of the stem, and containing one or 
more larval chambers. Examination of section under a compound 
microscope, however, reveals a condition similar to that described 
for C. solidaginis and C. s.-strobiloides. Each larval chamber is 
in reality the apex of a bud. The young leaves of the bud are 
closely applied to each other and their structure unaffected by the 
insect. As the gall developes the leaves do not unfold but assume 
a corky texture and in the fully mature gall their identity is 
almost lost. 

It is very evident that the larval chamber occupies a correspond- 
ing position in each of these galls. The insect prevents the 
elongation of the stem, thus causing the leaves of the apical 
bud to be bunched and reduced in size. The fact that the leaves 
of the Solidago reach the greatest development and those of 
the Quercus the least development is probably due to the char- 
acter of the plants. Of these three plants the growth of the 
Solidago is the most rapid while that of the Quercus is the slow- 
est. In Solidago the rapid growth may be sufficient to overcome 
the injury and cause the bunch of leaves ; in the Salix where the 
growth is not so rapid the leaves are smaller and more compact ; 
in the Quercus where the growth is slowest the bud never opens 
but becomes corky and the leaves gradually lose their identity. 

This work was pursued during the year 190 1-2 in the Zoolog- 
ical Laboratory of the Ohio State University under the direction 
of Professor Herbert Osborn to whom I am indebted for many 
valuable suggestions. 

LITERATURE. 

Only those references which were especially useful in preparing 
this paper are cited. 

1. Adler, Hermann, M. D., " Ueber den Generations — wech- 
sel der Eichen Gallwespen" Zeitschrift fur wissenschaftliche 

Zoologie. Bd. 35. Leipzig "Alternating Generations, a 

Study of Oak Galls and Gall Flies," translated by C. R. Stratton. 
Clarendon Press, Oxford. 

2. Ashmead, W. H. "A Bibliographical and Synonymical 
Catalogue of the North American Cynipidae with descriptions of 
new species." Transactions American Ent. Soc. Vol. XII, pp. 
291-304. 

3. Ashmead, W. H. " Synopsis of the North American Sub- 
families and Genera of Cynipidae." Trans. Amer. Ent. Soc. Vol. 
XIII, pp. 59-64. 

4. Ashmead, W. H. " On the Cynipidous Galls of Florida 
with descriptions of new species and Synopsis of the described 
species of North America." Trans. Amer. Ent. Soc. Vol. XIV, 
pp. 125-158. 

5. Bassett, H. F. "Description of several supposed new 
species of Cynips, with remarks on the formation of certain Galls. ' ' 
Proc. Ent. Soe. of Phil. Vol. II. No. 3, pp. 323-333. 



Ohio Naturalist. 



Plate 20. 







'WS^gP*^ 



Cook on Galls. 



276 The Ohio Naturalist. [Vol. II, No. 7, 

6. Bassett, H. F. "Descriptions of several new species of 
Cynips, and a new species of Diastrophus. " Proc. Ent. Soc. 
Phil. Vol. Ill, pp. 679-691. 

7. Beutenmueller, William. "Catalogue of Gall-producing 
Insects found within Fifty Miles of New York City, with Descrip- 
tions of their Galls, and of some new species." Am. Mu. of Nat. 
Hist. Vol. IV. No. 1. Art. XV. pp. 245-268. 

8. Fockeu, H. " Contributions a l'Histoire des Galls, Etude 
Anatomique de quelques especes. " Imprimerie & Librarie Ca- 
mille Robbe. Lille. 18S9. 

9. Fockeu, H. " Recherches anatomiques sur les Galls." 
Etude de quelques Dipterocecidies et Acarocecides." Imprimerie 
Typographique & Lithographiqua le Bigot Freres. 1896. 

10. Garmau, H. " The Phytopti and other Iujurious Plant 
Mites." Twelfth Rep. of State Ent. on Noxious and Beneficial 
Insects of the State of Illinois, pp. 123-143. 

11. Osten-Sacken, Baron R. " On the Cynipidaeof the N. A. 
Oaks and their Galls." Proc. Ent. Soc. of Phil. Vol. I. No. 3. 
pp. 47-72. 

12. Osten-Sacken, Baron R. "Additions and Corrections to 
title 11." Proc. Ent. Soc. of Phil. Vol. I. No. 8. pp. 241-259. 

13. Osten-Sacken, Baron R. " Contributions to the Natural 
History of the Cynipidae of the U. S. and of their Galls." Proc. 
Ent. Soc. of Phil. Vol. II. No. 1. pp. 33-49. id. Vol. IV. 
PP- 33i-38o. 

14. Osten-Sacken,- Baron R. "Two New North American 
Cecidomyia." Proc. Ent. Soc. of Phil. Vol. VI. pp. 219-220. 

15. Osten-Sacken, Baron R. "Biological Notes on Diptera 
(Galls on Solidago)." Trans. Am. Ent. Soc. Vol. II. pp. 

299-303- 

16. Packard, A. S. " Insects Injurious to Forest and Shade 
Trees." Fifth Rep. U. S. Ent. Com. 1890. 

17. Pergande, Theo. "The Life History of Two Species of 
Plant-Lice Inhabiting both Witch Hazel and Birch. ' ' Tech. Series. 
No. 9. U. S. Dept. Agr., Div. of Ent., 1901. 

18. Perris, E. " Galloides des Cecidomyies." Ann. de la 
Entom. de France, 4 e series, t. X. 1870. 

19. Riley, C. V. "The Grape Phylloxera, Phylloxera vas- 
tatrix, Planchon." 3rd, 4th, 5th, 6th, 7th, 8th Annual Reports 
of the Noxious and Beneficial and other Insects of the State of 
Missouri. 

20. Thomas, Cyrus. 8th Report of the State Ent. on the 
Noxious and Beneficial Insects of the State of Illinois. 1879. 

21. Walsh, B. D. "On the Genera of Aphididae found in 
the U. S." Proc. Ent. Soc. of Phil. Vol. I. No. 9. pp. 294. 

22. Walsh, B. D. " On Dimorphism of the Hymenopterous 
genus, Cynips, with an Appendix, containing hints for a new 
classification of Cynipidae and a list of Cynipidae, including 



Ohio Naturalist. 



Plate 21. 




Cook on Galls. 



278 The Ohio Naturalist. [Vol. II, No. 7, 

descriptions of several species inhabiting the Oak Galls of Illinois. ' ' 
Proc. Ent. Soc. of Phil. Vol. II. No. 4. pp. 443-500. 

22. Walsh, B. D. "On Insects, Coleopterous, Hymenop- 
terous, and Dipterous, inhabiting the Galls of certain species of 
Willows." Proc. Ent. Soc. of Phil. Vol. III. pp. 543-644; 
id. Vol. VI. pp. 223-288. 

EXPLANATION OF PLATES. 

In making the drawings, a Bausch and L,omb microscope and 
camera lucida were used. Figs. 1-7 were made with i-inch ocular 
and 1-5-inch objective. The diagrams of the galls were not made 
upon a definite scale. All other drawings were made with i-inch 
ocular and 2 3-inch objective. 



eviations : e. — epidermis. 


nu. 


— nutritive zone. 


end. — endodermis. 


0. e. 


— outer epidermis. 


f. — fibro-vascular bundle. 


P- 


— protective zone. 


1. c. — larval chamber. 


pa. 


—parenchyma. 


Cross section of leaf of Hicoria ovata. 






Salix cordata. 







I. 

2. 

3. " " Vitis vulpina. 

4. " " Ulmus americana. 

5. " " Acer saccharinum. 

6. " " Tilia americana. 

7. " " Ouercus alba. 

8. a. b. Phytoptus ulmi on Ulmus americana. 

9. a. b. " abnormis on Tilia americana. 

10. a. b. quadripes on Acer saccharinum. 

11. a. b. " acericola " " 

12. Schizoneura americana on Ulmus americana. 

13. a. b. Colopha ulmicola on Ulmus americana. 

14. a. b. Pemphigus ulmi-fusus on Ulmus americana. 

15. a. b. Hormaphis Hamamelis on Hamamelis virginiana. 

16. a. b. Phylloxera carya-avena on Hicoria ovata. 

17. a. b. c. " " fallax " " 

18. a. b. c. " " globuli " " 

19. a. b. " " spinosa " " 

20. a. b. " depressa " " 

21. a. b. " vastatrix on Vitis vulpina. 

22. a. b. c. d. Cecidomyia gleditsiae on Gleditschia triacanthos. 

23. a. b. pilulae on Quercus alba. 

24. a. b. verrucola on Tilia americana. 

25. a. b. Neuroterus irregularis on Ouercus macrocarpa. 
2.i. a. b. Callirhytis tumifica " alba. 

27. a. b. c. Holcaspis centricola " palustris. 

28. a. b. Amphibolips inanis " rubra. 

29. a. b. c. Dryophanta palustris " palustris. 

30. a. b. c. Callirhytis papillatus " sp. 

31. Longitudinal section of Cecidomyia Solidaginis on Solidago. 

32. " " salicis-strobiloides on Salix cordata. 

33. Callirhytis clavula on Quercus alba. 

a. Longitudinal section. 

b. Cross section. 

c. " " of leaf from b. 

d. " " of larval chamber from b. 

Note : — P. vastatrix was also collected on V. bicolor; C. pilulae was also 
collected on Q. rubra and Q. palustris. 



GALLS AND INSECTS PRODUCING THEM. 



Melville Thurston Cook. 



Part III. Lateral Bud Galls. 

In Part II of this series of papers I gave a discussion of apical 
bud galls. The lateral bud galls differ from the apical only in 
point of location ; therefore, this (Part III) may be considered a 
continuation of Part II. There is, however, considerable differ- 
ence in the galls dependent upon the order and genus to which 
the insect belongs and to the part of the plant which is attacked 
by the enemy. These differences may be summed up briefly as 
follows : 

( i ) Affection of the tip of the stem causing it to remain in its 
incipient condition and the leaves to remain aborted, instead of 
lengthening. This is well illustrated by the apical bud galls of 
Cccidomyia solidaginis Lw. on Solidago ; Cecidomyia salicis strobi- 
loides O. S. on Salix ; and Callirhytis clamda Fitch on Quercus 
alba. (Part II, Figs. 31, 32, 33.) In these cases we have two 
orders of insects represented but producing similar galls : this, as 
previously explained, is no doubt due to the fact that the insects 
affect corresponding parts of the host plant. 

(2) Affection of the tip of the bud causing it to remain short 
but to become large and globular. This is well illustrated by 
Hokaspis globulus Fitch (Fig. 34, a, b, c.) By collecting speci- 
mens of this gall in April or early part of May it is easy to 
demonstrate that the gall is in reality an enlargement of the stem 
part of the bud. The insect evidently deposits the egg in the 
apical part of the incipient stem. This causes the stem to enlarge, 
forming a globular body, but to remain so short as to form a 
sessile gall on the main stem. The bud scales are at first very 
prominent but gradually shrivel up and are lost, leaving a naked, 



42o The Ohio Naturalist. [Vol. Ill, No. 7, 

globular gall. At this late stage the only evidence that we have 
of its bud origin is its location at the node of the main stem. 
The transition from bud to gall occurs verj^ early, before there is 
any differentiation of the parenchyma tissue ; examination of the 
structure of the gall fails to show any stem characters but does 
show the Cynipidous gall character described in Part I of this 
series. 

(3) The third type of the bud gall is illustrated in Andricus 
seminator Harris (Figs. 35, a, b, and 36, a, b.) Ashmead* refers 
to this as a flower gall. It is not difficult to demoustate that this 
gall is a true, compound bud gall, but whether it is a flower or 
leaf gall is not so easily determined. The strongest evidence of 
its bud character is its location at the node of the stem and the 
presence of the leaf scales at its base. The writer gathered and 
dissected a large number of galls of various ages and is confident 
that this is a true compound bud gall. In Figure 35 a, we have 
a short twig with three buds, one of which was attacked by the 
insect ; the other two buds remained unaffected. Around the 
base of the gall are four well-defined bud scales. In Figure 35 
1), two buds were affected ; one of these has been removed show- 
ing the scar where it was attached and also exposing the back 
side of the compound gall formed from the other bud. A great 
many galls of various ages were dissected ; the younger ones 
showing the bud scales and the older ones showing the well- 
defined scars by which it was easy to trace the number of buds 
affected. Careful observations were made in hopes of finding a 
gall which would show whether this was a leaf or flower bud, but 
without success. However, from a careful microscropic examin- 
ation of a number of galls I am inclined to consider it a leaf bud, 
in which each leaf becomes a single gall of the large cluster and 
in which the incipient stem remains short. The microscopic 
examination of the single galls ( Fig. 36, a, b) shows that each 
gall contains at least one (and usually only one ) fibro- vascular 
bundle which in most cases is very much atrophied and in some 
cases so much reduced as to be very indistinct. The writer 
considers the fibro- vascular bundle as the mid-rib of the modified 
leaf and the cottony part of the gall as the mesophyll part of the 
leaf. This gall does not show the four zones which are charac- 
teristic of the cynipodous galls as pronounced as other galls 
which we have examined, but this point will be discussed in a 
later paper. 

(4) The fourth type of gall is illustrated by a cecidomyid gall 
( Fig 37) found upon Acer negundo in which the bases of the 
petioles of a number of leaves from the same bud are enlarged, 



*Ashmead. Wm. H.: "On the Cynipidous Galls of Florida, with descriptions of new 
species and synopses of the described species of North America." Trans. Anier. Knt. Sec. 
Vol. XIV. pp: I2S-I2S 



May, 1903.] Galls and Insects Producing Them. 421 

thus forming a bulb-like compound gall. On the inner surface 
of the base of each petiole is a cavity containing the larva. The 
stem remains short but the outer leaves are fully developed in 
most cases. 

(5) Pachypsylla celtidis-gemma Riley (Fig. 38) is evidently a bud 
gall very similar to the preceding. Only advanced stages of this 
gall were collected, and therefore its development could not be 
observed. From the specimens collected it appeared that each 
scale and undeveloped bud formed a pocket for the insect, there 
being a single insect under each scale. 

CONCLUSIONS. 

Bud galls are subject to considerable variation due to the fact 
that they are produced by insects of different orders and that 
these insects attack different parts of the buds and different 
tissues in these parts. In all cases except the fourth the demands 
of the insect are so great as to cause a very pronounced change in 
the bud. In the fourth the modifications are not so pronounced 
as in the other four types. 

Part IV. Stem Galls. 

Stem galls, according to my definition, include only those galls 
which cause a swelling of the stem and with the larva placed in 
or near the center, thus affecting the stelar and fibro-vascular 
parts of the stem. This definition ma}' not be as broad as it 
should be, but I hesitate to make it include other forms until I 
have had an opportunity to make a more careful examination of 
the questionable forms. The fact that such galls as H. globulus 
(Fig. 34, a, b, c), which is frequently mentioned as a stem gall,- 
are in reality bud galls, leads me to be doubtful of the origin of 
galls which have similar locations. Many of the so-called stem 
galls may be in reality bud galls and this point can be determined 
only by a study of their development and structure. 

Some galls occur on both leaves and stem, but in these cases 
the gall affects only the outer layers of the cells of very young 
twigs and these cells at this time resemble the leaf cells in both 
structure and functions. Phylloxera carya-spinosa Shimer (Part 
I, Fig. 19) and Phylloxera caryae-caulis Fitch (referred to in 
Part V) are good examples of leaf galls affecting stems. 

The Tepidopterous galls are usually stem galls and may be 
either solid or hollow and are most common on Solidago. In 
studying such galls it is necessary to examine first a normal stem. 

The stem of Solidago. (Fig. 39) shows the ordinary dicotyledo- 
nous character. The epidermal cells (e p) are firm and rather 
hard. Just below these cells is the parenchyma zone (pa) of 
closely-fitted cells and few intercellular spaces. Below the par- 
enchyma zone are the fibro-vascular bundles (p. v. b.), which 



422 The Ohio Naturalist. [Vol. Ill, No. 7, 

contain a large amount of woody, fibrous tissue. Inside the zone 
of fibro-vaseular bundles and forming the axis of the stem, is the 
stelar (st) made up of large parenchyma cells. 

In Tiypcta solidaginis (Fig. 40) a solid globular gall on the 
stem of Solidago, we find the walls of the outer parenchymatous 
cells much thickened and numerous large intercellular spaces 
which are not characteristic of the unaffected stem ( Fig. 39). 
The fibro- vascular bundles (f. v. b. ) are spread out and flattened, 
the sclerenchyma tissue and tracheary tissue being reduced and 
the fibrous tissue increased in amount. The parenchyma tissue 
of the stelar (st) part of the gall is increased in amount and the 
size of the cells reduced. This tissue is undoubtedly very active 
and well supplied with nutrition for the larva. Throughout the 
tissue are tubes (tu) lined with cells smaller than the parenchj-ma 
cells, brown in color, and not affected by haematoxylin stain. 
These tubes are usually associated with small bundles of fibrous 
tissue and are probably important factors in the nutrition of the 
larva. They were not found in sections of normal stem of corre- 
sponding age. 

In Gelechia gallae-solidaginu Fitch (Fig. 41) an elongated, 
hollow gall on Solidago, we find the parenchymatous tissue (pa) 
near the surface increased in amount, the cells larger and the 
walls thicker than in an unaffected stem, but no intercellular 
spaces such as are found in T. solidaginis. The fibro-vascular 
bundles (f. v. b. ) undergo comparatively little change, becoming 
slightly flattened and thinner and with a reduction of the firmer 
fibrous tissue. The larva chamber (1. c.) of the gall is lined with 
a few layers of small parenchymatous cells (st) and is the stelar 
part of the stem. This parenchymatous tissue is udoubtedly 
used for food. 

In Cecidomyia rigidae O. S. (Fig. 42) an elongated, hollow gall 
common on Salix discolor, usually near the tips of the twigs, we 
find considerable modification of the normal stem structure. From 
the examination of a number of specimens it is very clear that the 
enlargement of the stem is due to two factors : the formation of 
large intercellular spaces near the surface, similar to those in T. 
solidaginis (Fig. 40), and the formation of the larval chamber 
(1. c.) in the stelar part of the stem. The parenchymatous tissue 
lining the chamber is made up of cells very much smaller than 
those in an unaffected stem. 

The Fepidopterous galls on the young stems of Acer neguudo 
and Coleopterous galls on Rubus villosus were examined but no 
new points presented. I was unable to secure satisfactory speci- 
mens of stem galls of Cynipidae. 

Although the study of stem galls was in many respects unsat- 
isfactory, I feel justified in giving the following brief conclusions : 



May, 1903.] Galls and Insects Producing Them. 423 



CONCLUSIONS. 

1. Stem galls show less variations than any other group of 
galls, although they may be produced by insects from widely 
different orders. This is undoubtedly due to the fact that the 
various insects attack corresponding parts of the host plants. In 
proof of this fact, it will be noticed that all these insects deposit 
the egg within the tissues of the host plant and not on the surface. 

2. The galls in general show an increase of parenchyma below 
the epidermis, either a thickening of cell walls or a development 
of intercellular spaces, a flattening of the fibro-vascular bundles, 
an increase of parenchyma tissue in stelar part of stem and a 
decrease in size of same. 

Part V. Development oe gales. 

A very large amount of material was collected for this paper 
and great difficulty was experienced in getting the extremely 
young stages because of the fact that young specimens were diffi- 
cult to recognize and identify. The material was carefully killed 
in either Fleming's solution or chromo- acetic, passed through the 
alcohols, imbedded in paraffin, sectioned on a Zimmerman micro- 
tone and stained in haematoxyliu. 

The galls will be considered in the same order as in Part I of 
this series. A consideration of the leaf structure is unnecessary 
since that was considered in Part I. 

I. GALLS OF ACARINA. 

Young galls of Phytoptus quad ri pes (Fig. 43), P. abnormis (Fig. 
44), and P. acericola (Fig. 45) were studied, and all show the 
same developmental characters. The leaf becomes slightly pitted 
on one side (usually the lower) and a corresponding elevation is 
formed on the upper surface. This gradually enlarges until the 
more or less spherical gall is produced. In P. abnormis the 
spherical gall soon assumed an elongated form. The character- 
istic cell structure of the leaf is lost and the cells become very 
irregular in shape. The elongated character of the cells just 
beneath the outer epidermis appears at a later period of the devel- 
opment. At first the inner surface of the gall is perfectly smooth, 
but very soon masses of cells are formed and project into the 
cavity (Figs. 43 and 45). At about the same time trichomes 
begin to develop from the inner epidermis (Fig. 44) and project 
into the cavity. These trichomes grow very rapidly and almost 
fill the entire cavity. 

In the very young galls no fibro-vascular bundles are formed, 
but in the older galls small bundles of fibrous tissue are numerous. 

The first effect of the insect attack is undoubtedly to cause an 
increase in the number of cells, which is an effort on the part of 
the plant to heal the wound produced by the repeated puncturing 



4^4 The Ohio Naturalist. [Vol. Ill, No. 7, 

of the cells by the parasite. Since the parasite continues its 
attack upon different cells and the plant makes the repeated effort 
to heal the wound, we have the very active production of cells. 
The parasite making its attack upon one side of the leaf, causes 
the unequal growth resulting in a cavity. The increase in size 
of the gall causes a different tension upon the inner and outer 
surfaces and results in the elongation of cells near the outer sur- 
face as described in Part I. 

When the galls first appear they are single, but in a very short 
time others are formed just outside the first, thus forming a 
cluster. 

In Erineum anomalum (Figs. 47, 48, a, b), occurring on leaves 
and petioles of walnut, we find a condition similar to that of the 
Phytoptus galls except that the parasite is on a free surface 
instead of in a partly closed cavity. I was able to secure a very 
complete series of this gall. The first indication of the gall on the 
petiole or rib of a leaf is the increase in the amount of parenchyma 
tissue between the epidermis and fibro-vascular bundles. The 
physiological character of this tissue is also changed to some 
degree, since the cells are not so easily stained with haematoxylin, 
have rather thick walls, and contain a considerable quantity of 
tannin. The epidermal cells now begin to form trichomes (Fig. 
47). The parenchyma tissue and trichomes both increase in quan- 
tity, the walls of the cells become thinner (Fig. 48, a, b), and the 
deeper parenchyma tissue gradually loses its tannin, while the 
outer cells retain it in great quantities. 

These galls always occur over a fibro-vascular bundle and are 
apparently closely associated with them. These bundles become 
modified to some extent. 

The origin and development of these galls is the same as in the 
Phytoptus galls except that the parasite works upon the exposed 
surface instead of in a cavity. The fact that one produces a 
cavity lined with trichomes while the other produces a protuber- 
ance covered with trichomes, is probably due to the fact that the 
latter is so closely associated with the fibro-vascular bundle which 
prevents the curvature but causes the rapidly-formed cells to 
swell outward into a protuberance. 

2. GALLS OF THE APHIDIDAK. 

In the Aphididae galls we have a condition very similar to that 
just described for the Acarina galls except that the shape of the 
galls are far more definite and they show a higher degree of 
development. Trichomes are not so numerous and masses of cells 
projecting into the larval chamber as described for Phytoptus 
galls are very rare. In the youngest galls the cell structure of 
the leaf is modified, resulting in the formation of a large number 
of small, irregular cells, the same as in the Acarina galls. As the 



May, 1903.] Galls and Insects Producing Them. 425 

galls grow older the cells near the outer epidermis become elon- 
gated as in the Phytoptus galls. 

In Pemphigzk ulmifusus (Walsh) Oestland (Fig. 49, a, b) on 
U. Americana, we have the gall originating first as a fold in the 
leaf which becomes developed into a conical structure. The struc- 
ture of the gall shows that the characteristic structure of the leaf 
is at first modified into a large number of small, irregular-shaped 
cells (Fig. 49, b). The tendency for the cells near the outer 
surface to elongate parallel to the surface begins with the further 
development of the gall. In the very young galls the tannin is 
in very small quantities, but increases as the gall grows older. 

In Colopka ulmicola Fitch (Fig. 50, a, b) we have a condition 
almost identical with P. ulmi-fusus. The gall first appears as a 
slight fold in the leaf and later develops into the characteristic 
cockscomb gall. The cell structure is the same as in P. ulmi- 
fusus. 

In Phylloxera carya-fallax Riley (Figs. 51, 52) on H. ovata, I 
secured the youngest galls possible to detect and identify. These 
galls showed a slight projection from both surfaces of the leaf, 
but at first the gall was not so conical as at a later period of its 
development. However, the youngest galls showed the charac- 
teristic structure described in Part I of this series. The first 
effect of the parasite attack appears to be the formation of a large 
number of irregular cells. The arrangement of these cells is the 
same in the young gall as in the more mature, but the fibro-vas- 
cular bundles of the older specimens were not observed in the 
young galls. 

I was not so successful in securing young specimens of P. 
c.-globuli Walsh (Fig. 53), but, so far as I was able to observe, 
the line of development coincided with P. c.-fallax. However, 
the upper wall of the gall is at first very thin and grows in thick- 
ness as the gall approaches maturity. 

Phylloxera ea>ya-eaidis Fitch of Hickory ovata was studied very 
carefully from a very complete series of specimens. The material, 
especially the younger galls, did not cut well, and so was not 
satisfactory for drawings. However, the development and struc- 
ture were of the typical Phylloxera type corresponding very 
closely with that just described for P. c.-fallax. The only marked 
peculiarity was the close association with fibro-vascular bundles, 
the galls always occurring on very young green twigs, on mid-rib 
or on prominent veins of the leaf. 

Pemphigus populi-transversus Riley (Figs. 55, a, b, and 56, a, b) 
and P. p.-caulis Fitch (Figs. 57, a, b, c, and 58, a, b, c) of the 
Populus are galls growing on the petiole ; the former at some 
point between the blade and stem, the latter at the base of the 
leaf. In both cases the attack is made from the outside, the same 
as in other Aphididae galls and in the Acarina galls. A careful 



426 The Ohio Naturalist. [Vol. Ill, No. 7, 

study of an excellent series of both galls shows a cell structure 
and development very similar to other Aphididae galls ; i. e., a 
large number of small, irregular cells. In P. p.-transversus (Fig. 
55, a, b) the gall originates as a swelling on the petiole and 
within this swelling is a large cavity opening to the outside 
through a slit. In the P. p.-caulis the same condition is true 
but the attack of the insect causes a one-sided growth, resulting 
in the petiole being twisted at right angles to the blade (Figs. 
57, a, b, c, and 58, a, b, c). 

A careful examination of the cell structure of P. p.-transversus 
(Fig. 56, a, b) and a comparison with the unaffected petiole (Fig. 
54, a, b) indicated a very rapid growth, resulting in the very 
large number of small, irregular cells. The character of the 
young and of the mature gall was practically the same, and not 
different, as in the more highly developed galls of other orders. 
The fibro-vascular bundles were very slightly affected. 

P. p.-caulis showed the same cell structure and development, 
and, judging from these points alone, one would be unable to 
separate these two galls. 

3. GALLS OF PSYLLIDAE. 

In Pachypsylla celtidis-mamma Riley (Figs. 59 and 60, a, b, c) of 
the Celtis occidentalis the youngest galls did not show a cavity, 
but showed a modification of the leaf by which there is formed a 
large number of small, irregular cells which can be readily sepa- 
rated into two zones ; the upper made up of small, and the lower 
of somewhat larger cells (Fig. 59). I was unable to secure speci- 
mens intermediate between this stage and a later stage, showing 
the true form of the gall (Fig. 60, a, b, c) The youngest galls, 
showing the true form, exhibited four well-defined zones: (1) 
epidermis, (2) zone of large, irregular-shaped cells, (3) zone of 
elongated cells, (4) zone of irregular-shaped cells next to the 
larval cavity. Adjacent to zone (3), but derived from zones (2) 
and (4), are cells which even in very young galls show schleren- 
chyma characteristics. As the gall approaches maturity this 
tissue increases until in the mature gall it may be found in great 
abundance. This gall is undoubtedly the most highly developed 
of any of the Hemiptera galls which I have studied. 

4. GALLS OF CECIDOMYIA 

Although I have a large number of Cecidomyia leaf galls, I 
have succeeded in getting a series of only two species. Since the 
Cecidomyia show by far the greatest variation in structural char- 
acters and the smallest number of typical group characters, two 
species are not sufficient to draw a very definite conclusion. 

In Cecidomyia gleditsiac O. S. (Fig. 61, a, b) the two halves of 
the leaflet never have an opportunity to unfold, but there is a 



May, 1903.] Galls and Insects Producing Them. 427 

growth of cells allowing the leaflet to enlarge and form the larval 
chamber between the two halves. The cells are at first normal, 
but gradually lengthen in an axis at right angles to the mid-rib. 
This can be readily observed by comparing the section of the very 
young gall (Fig. 61, a, b) with the section of the mature gall 
(Part I, Fig. 22). 

In Cecidomyia verrucola O. S. (Figs. 62 and 63) the youngest 
showed a condition in which the mesophyll part of the leaf was 
reduced or entirely removed by the larva. The upper epidermis 
and palisade cells, the lower epidermis and cells next to it, form 
the upper and lower walls of the larval chamber while the inter- 
mediate mesophyll is removed. The inner layers of cells, i. e., 
the cells next to the larval chamber, now grow and divide very 
rapidly, gradually filling almost the entire cavity and reducing 
the size of the chamber (Part I, Fig. 24). At the same time the 
gall is increasing rapidly in size. 

5. GALLS OF THE CYNIPIDAE. 

Although a large amount of material was collected, only three 
species were sufficiently complete to enable a satisfactory study. 
However, several mature galls of species not described in Part I 
of this series were examined, and all agreed with the statements 
made concerning the general structural character of this group 
of galls. 

CallirkyHs papillatus O. S. (Fig. 64) was especiallj- difficult to 
collect because of its very small size and close resemblance in 
external appearance to other small Cynipidous galls. Examina- 
tion of young Cynipidous forms, which I am reasonably certain 
belong to this species, show all the zones in contact (Fig. 64). 
As the gall develops the protective zones and parenchyma zones 
separate but remain connected by elongated parenchymatous cells 
(Parti, Fig. 30). 

Dryophanta palustria O. S. (Fig. 65, a, b) appears as the leaves 
unfold from the bud. The youngest galls collected were not over 
two millimeters in diameter but showed the four zones well devel- 
oped, with the second and third zones in contact, thus verifying 
the views expressed in Part I. The cells of the innermost, or 
nutritive, zone were large and very granular. Evidently this zone 
was almost completely reduced by the larva in the specimen from 
which Fig. 29 of Part I was drawn. In the next, or protective, 
zone the cell walls were very thick. In the parenchyma zone the 
innermost cells were small and numerous and the walls were thin, 
and in both cases the long axis of the cells were at right angles 
to the surface of the gall. As the gall grows older the intercellu- 
lar spaces may become prominent among the cells of the paren- 
chyma zone (Fig. 65, b). Careful examination of a large number 
of specimens gave conclusive proof that the separation occurs 



428 The Ohio Naturalist. [Vol. Ill, No. 7, 

between the protective and parenchyma zones, thus leaving the 
two inner zones as a small sphere rolling free within the larger 
sphere which is formed by the two outer zones. 

In Diastrpphus siminis Basset (Figs. 66, a, b; 67; 68, a, b, c, d; 
69 ) we have a Cynipiclous gall occurring on Nepeta glechoma. I 
secured a very complete series of this gall and made a very careful 
study of its development. In the youngest gall (Fig. 66,^, b) 
we have the cell character of the leaf transformed into a mass of 
small, irregular cells which can be readily divided into two zones, 
the outer of which has the larger cells. At this time the cells are 
very compact, but as the gall grows older intercellular spaces are 
developed, the entire structure becomes loose and spongy and the 
cells become larger. 

As the galls grow older a well-defined zone of flattened cells 
is developed in the parenchyma near the epidermis, and fibro-vas- 
cular bundles ( f . v. b. ) are developed at right angles to the sur- 
face (Fig. 67). Up to this time the cells are small, irregular and 
compact. The epidermis (ep) and parenchyma (pa) zones are 
well defined, but the distinctiion between protective and nutritive 
zones cannot be made. 

As the gall grows older a cleavage plane is formed in the paren- 
chyma just inside the zone of flattened cells (Fig. 68, a). A 
careful examination of the parts thus cut off and surrounding the 
larval chamber (1. c. ) shows two well-defined zones which corre- 
spond to the nutritive and protective zones described in Part I. 
At this time there is no marked difference in the amount of food 
supply of the tw r o zones. In the outer part formed by this cleav- 
age plane we have the parenchyma (pa) and epidermal (ep) 
zones (Fig. 68, c). Connecting the parenchyma and protective 
zones we find fibro-vascular bundles ( f . v. b. ) surrounded by par- 
enchyma cells (Fig. 68, d). The character of these connecting 
strands is very similar to that described for H. centricola (Part I, 
Fig. 27) and A. inanis (Part I, Fig. 28), but contains more par- 
enchyma tissue than either. However, the parenchyma cells are 
not so elongated as in C. papillatus (Part I, Fig. 30). As the 
gall grows older the cells of the protective zone become clear and 
the cell walls of the nutritive zone gradually thicken (Fig. 69), 
many undergoing complete degeneration, while others assume the 
character of the sclerenchyma. 

CONCLUSIONS. 

1 . All conclusions given in Part I are emphasized by the study 
of the development of the galls. 

2. In the formation of all leaf galls except the Cecidomyia 
galls, the normal cell structure of the leaf is first modified by the 
formation of a large number of small, compact, irregular-shaped 
cells. In the galls of Acarina and Aphididae this is followed by 



May, 1903.] Galls and Insects Producing Them. 429- 

a development of trichomes, especially the former. In all galls 
the mesophyll is subject to the greatest modification. Many 
small fibro vascular bundles are formed in this modified mesophyll. 

3. The Acarin may be considered the lowest group of galls, 
the Aphidid the next higher, the Cecidomyia galls the next 
higher, and the Cynipidous galls the highest. However, many 
of the Cecidomyia galls are lower than the Aphidid galls. 

4. The galls of Acarina and Aphididae show the greatest 
resemblance. In these cases the method of attack is very similar 
and is first directed against the epidermal or adjacent layer of cells. 

5. In some of the Cecidomyia galls (e. g. C. verrucola) the 
larva appears to make its entrance into the mesophyll before there 
is any pronounced modification of the cell structure. However, 
the Cecidomyia galls are too varied and the study too incomplete 
to make a positive conclusion. 

6. Both Adler and Fockeu consider that after the first stages 
of formation, the gall becomes an independent organism growing 
upon the host plant. This is probably true in the highly devel- 
oped galls of Aphididae, Cecidomyia and Cynipidae, but the 
writer is very doubtful if this is true of the less complex galls of 
Acarina, Aphididae and Cecidomyia. 

This work was pursued during the year 1902-03, in the Biolog- 
ical Laboratory of DePauw University, but was under the super- 
vision of Professor Herbert Osborn, of the Ohio State University, 
to whom I am indebted for many valuable suggestions. I am 
also indebted to two of my former students, Miss S. Emma Hick- 
man and Miss Margaretta S. Nutt, for aid in preparing slides and 
making drawings. Drawings made by these two ladies are marked 
with their initials. I also wish to express my thanks to my many 
friends who have called my attention to, or have collected material 
for, these investigations. 

LITERATURE. 

New literature will not be cited at this time, but a more com- 
plete list will be given in connection with later papers upon this 
subject. 

EXPLANATION OF PLATES. 

In making the drawings a Bausch & I,omb microscope, with 
No. 2 ocular and }4 objective, and a B. & L camera lucida were 
used. The drawings are, therefore, larger than those used in 
Parts I and II, and the reduction not so great. The diagrams 
are not made upon a definite scale. Drawings 34, a, b, c ; 35, a, 
b; 37, 38, 55, a, b ; 57, a, b, c, and 58, a, b, c, were made from 
nature, and are very little smaller than the original. The num- 
bering of the drawings is continuous with Parts I and II. 



43° The Ohio Xaturalist. [Vol. Ill, No. 7, 



ABBREVIATIONS. 

ep. — epidermal zone. 

pa — parenchyma zone. 

pr. — protective zone. 

nu. — nutritive zone. 

f. v. b.— fibro-vascular bundles. 

34, a. Bud of Hicoria ovata. 

34. b. c. Holcaspis globulus on H. ovata. 

35, a. Andricus semiuator gall and two buds on O. alba. 

35. b. Andricus seminator gall and bud scar on Q. alba. 

36, a, b. Section of Andricus seminator gall on Q. alba. 
37- Cecidomyia gall on A. negundo. 

38. Pachsylla c -gemma on C. occidentalis. 

39. Cross section of stem of Solidago. 

40. Trypeta solidaginis on Solidago. 

41. Gelechia gallae solidaginis on Solidago. 

42. Cecidomyia rigidae on Salix. 

43. Phytoptus quadripes on A. saccharinum. 

44- abnormis on T. Americanura. (Two larval chambers.) 

45- " acericola on A. saccharinum. 
46. Petiole of Juglans nigra. (Cross section.) 

Erineum anomalum on J. nigra. Young gall 

48, a. b. Erineum anomalum on J. nigra. (Mature gall.) 

49, a, b. Pemphigus ulmi-fusus on U. Americana. 

50, a, b. Colopha Ulmicola on U. Americana. 

51, Phylloxera carya-fallax on H. ovata. 

52, '• 

53- carya-globuli on H. ovata. 

54, a, b. Cross section of petiole of Populus monilifera. 

35, a. Pemphigus populi-transversus on petiole of P. monilifera. (Yotmg gall.) 

55, b. Same in section. 

56, a. P. p.-transversus. Part of gall near opening into larval chamber. 

56, b. P. p.-transversus. Section back of chamber and showing one fibro-vascular 

bundle of the petiole. 

57, a. P. p.-caulis. Young gall ; ventral surface. 
57. b. Young gall; dorsal suiface 

57. c- " Young gall ; open. 

58, a. Ventral surface. 

58, b. Dorsal surface. 

Open. 

59, Paehypsylla celtidis-mamma on C. occidentalis. (Young gall. 
£0, a. P. c. -mamma. Diagram. 

60, b. Section of dorsal part. (2 and 3.) 

60, c. Section of ventral part. (3 and 4.) 

61, a. b. Cecidomyia gleditsiae on G. triacauthos. 

62, verrucola on T. Americana. 1 Young gall.) 
63. 

64. Callirhytis papillatus on Q. palustris. 

65. a. b. Dryophanta palustris on O. palustris. 

66. a, b. Diastrophus siminis on X. glechoma. 

67. •• - •• ' •• 

68. a. " " Diagram. 

68, b. " Nutritive and protective zones. 

68, c. Epidermal and parenchyma zones. 

■68, d. " Strand connecting protective and parenchyma zones. 

•69- " Nutritive zone in gall almost mature. 



May, 1903.] Galls and Insects Producing Them. 



Ohio Naturalist. 



43i 
Plate 13. 




Cook on "Galls and Insects Producing Them. 




£* 



Cook on "Galls and Insects Producing: Them. 



May, 1903.] Galls and Insects Producing Them 



Ohio Naturalist 




Cook on "Galls and Insects Producing Them." 



434 



Ohio Naturalist. 



The Ohio Naturalist. 



[Vol. Ill, No. 7, 
Plate 16. 




Cook on " Galls and Insects Producing Them. 



May. 1903.] Galls and Insects Producing Them. 

Ohio Naturalist. 




Cook on "Galls and Insects Producing 1 Them. 



436 

Ohio Naturalist. 



The Ohio Naturalist. 



[Vol. Ill, No. 7, 
Plate iS. 




Cook on -Galls and Insects Producing Them." 



[Reprinted from Ohio Naturalist, Vol. IV, No. 6.] 



GALLS AND INSECTS PRODUCING THEM. 



Melville Thurston Cook. 



Part VI. Flower and Fruit Galls. 

Galls affecting flowers and fruits are not so abundant as those 
affecting leaves, but in many cases the insect which produces 
flower or fruit galls also produces leaf galls. No sharp line of 
distinction can be drawn between flower and fruit galls, since the 
gall may form and mature without indication of fruit or ma3 T 
form in the flower and mature as the fruit develops. Thus far I 
have collected five species of flower and fruit galls representing 
three orders of insects. 

I. GALLS OF THE ACARINA. 

Phytoptus sp. — on Euphorbia corallata L. (Figures 70 ; 71a, 
b ; 72a, b). This mite produces galls on both leaf and flower. 
The structure of the gall is the same in both cases and is identi- 
cal with Phytoptus galls, previously described in Part I, (Figures 
8-1 1). All my specimens of this gall were well advanced. The 
structure of the leaf of E. corallata (Fig. 70) is typical. When 
attacked by the Phytoptus the leaf becomes very much modified 
by thickenings, ridges and convolutions (Figures 71a, b). The 
palisade cells divide so that it is impossible to distinguish them 
from the mesophyll, and the intercellular spaces are obliterated as 
the result of the rapid cell division. The new cells are small and 
very rich in protoplasm, but gradually become filled with tannin 
as the gall approaches maturity. The tannin first forms in the 
outer and most exposed cells of the gall while the inner layers of 
cells retain their protoplasm very late. The Phytoptus restricts 
its attacks to these inner and more protected parts. From a study 
of these galls it is apparent that the Phytoptus is not working on 

-Contributions from the Department of Zoology and Entomology, Ohio State Univer- 
sity, under the direction of Prof. Herbert Osborn, No. 17. 



n6 The Ohio Naturalist. [Vol. IV, No. 6, 

all parts of the gall at the same time, but gradually moves out- 
ward over the surface of the leaf, thus increasing the size of the 
gall and drawing its food supply from the newer part thus formed. 
When the attack is made upon the flower we have a mass of 
distorted tissue which is structurally the same as that produced 
in the leaf gall (Figures 72a, b). The floral envelopes are the 
first to suffer from the attack, the ovary with its contents is the 
next greatest sufferer, while the stamens are frequently unaffected. 
It is evident that the attack upon the flower must be made very 
early in order to cause complete destruction. Very frequently 
the floral envelopes will be very much deformed and the ovary 
and the stamens very slightly affected. In other cases the ovary 
will be very much enlarged and its chambers practically obliter- 
ated. It is evident that the attack upon the ovary must be made 
very earl}' to produce a great deformity. The partial immunity 
of the stamens is probably due to their being very nearly mature 
before the opening of the bud. 

2. GALLS OF CECIDOMYIA. 

Cecidomyia anthophila O. S. — on Solidago cauadense L. (Figs. 
73a, b), makes the attack early and completely prevents the open- 
ing of the bud. The gall is in the form of a hollow cone. The 
transformation is so complete that the location is the only evidence 
that the gall is produced from a flower bud. A section of the 
gall shows the nutrient layers of the cells next to the larval 
chambers, large parenchyma cells near the outer epidermis, and a 
number of rather weak fibro- vascular bundles. 

Cecidomyia sp. — on Ratibidapinnata Barnhart (Figs. 74a, b, c). 
The entire bud is transformed into a gall with the larva in a 
chamber in what was originally the ovary. All the floral parts 
have become modified and united to form the gall. A section of 
the gall (Fig. 74c) shows that the cells are more uniform in size 
than in the preceding galls and that the fibro-vascular bundles 
are practically obliterated. 

Cecidomyia sp. — on Prunus virginiana L,. (Figs. 75a, b). My 
specimens of this gall were mature. I am unable to say at what 
time the gall originates, but it reaches its maturity with the fruit. 
The gall is somewhat larger than the fruit, but otherwise resem- 
bles it closely. The larva makes its exit through an opening at 
one side of the stem. The larval chamber is very large, thus 
giving the gall a bladder-like character. The cuticle is well 
developed and the parenchyma cells below it are very large, while 
the cells next to the larval chamber are much smaller. Weak 
fibro-vascular bundles are also present. The wall of the gall 
(Fig. 75b) is much thicker than the wall of the fruit at this time 
(Fig. 75a), and parenchyma cells are much larger. The charac- 
teristic stone (sclerenchyma) of the fruit is never developed in 
the gall. 



April, 1904.] Galls and Insects Producing Them. 117 



3. GALLS OF LEPIDOPTERA. 

I gathered a number of Lepidopterous galls on Rudbeekia 
laciniata L. which I was unable to determine. These galls occur 
on both leaf and flower and are very large and fleshy. In fact 
they were so fleshy and juicy that it was very difficult to secure 
sections. The parenchyma cells were very large, and small fibro- 
vascular bundles were numerous. The larval chambers were 
numerous and each contained a single larva or pupa. In my 
specimens the larvae were far advanced, many of them in the pupa 
stage, but the cells next to the chambers were very rich in food 
supply. 

Part VII. Root Galls. 

Amphibolips radicola Ashm. (Figs. 76a, b). — on Quercus alba 
Iv. was the only root gall that I collected. The galls were borne 
just under the surface of the ground at about the point of transi- 
tion from stem to root. The)' were produced in great numbers 
and so closely packed together as to assume the shape of figs. 
Those nearest the surface of the ground and therefore slightly 
exposed to the light were of a rich, red color, while those deeper 
in the ground were almost white, slightly tinged with yellow. 
Each gall contained from one to five larval chambers. The 
younger galls showed four zones well defined (Fig. 76a). The 
inner or nutritive zone was thick and the cells contained abund- 
ance of protoplasm. The protective zone was thin and the cells 
fibrous in character rather than sclerenchymatous. The paren- 
chyma zone was thick and composed of large parenchyma cells. 
The epidermal zone was relatively thick and the cells firm. As 
the insects approach maturity the nutritive and protective zones 
are entirely destroyed (Fig. 76b). The insect eventually makes 
its escape through an opening in the side of the gall. 

Part VIII. Histology of Galls. 

Many of the histological characters of galls have been referred 
to in the preceding parts. This part has been introduced at this 
time for the purpose of adding a few additional facts which were 
not clear at the time of the writing of the preceding parts. 

A. Internal Stiucturcs. 

I. galls of acarina. 

These galls have been sufficiently discussed and need very little 
attention at this time. In general these galls may be thrown 
into three groups : (1) Those galls in which there is very little 
distortion, but a modification of the epidermis, as in the case of 
the Phytoptus on the beech ; (2) Convolutions of the parts as in 
the case of P. ulmi (Fig. 8), P. abnormis (Figs. 9, 44), P. quad- 



n8 The Ohio Naturalist. [Vol. IV, No. 6, 

ripes (Figs. 10, 43), and P. acericola (Figs, n, 45). These con- 
volutions result in the formation of a more or less well defined 
cavity, and trichomes are developed in great abundance in the 
younger stages ; (3) Thickening of the parts which become cov- 
ered with an abundant growth of trichomes as in the case of E. 
anomalum (Figs. 47, 48). 

The Phytoptus galls show two fairly well-defined zones, the 
outer made up of rather large cells and the inner of much smaller 
cells, which are very rich in protoplasm and which supply nour- 
ishment for the young animal (Fig. 77). As the galls approach 
maturity the protoplasm disappears, first from the outermost cells 
and lastly from the cells on the inner surface. As the protoplasm 
disappears the tannin accumulates in great abundance (Fig. 78). 

2. GALLS OF THE APHIDIDAE. 

Many of the Aphididae galls produce trichomes which soon 
disappear. At first all the cells contain protoplasm and divide 
rapidly, but as the galls approach maturity the tannin increases 
in abundance. 

Schizoneura americana Riley (Fig. 12), Colopha ulmicola Fitch 
(Fig. 13), and Hormaphis hamamelis Fitch (Fig. 15) have been 
considered in Part I. 

In Pemphigus populi-transversus (Figs. 55, 56) and P. p.-caulis 
(Figs. 57, 58) the thickness of the walls of the galls is much 
greater than any other members of this famity and the cells are 
more uniform in character. These galls are especially well sup- 
plied with fibro-vasular bundles and are very dense. 

In P. vagabundus (Fig. 112) we have a gall in which many of 
the cells are elongated similar to C. ulmicola and H. hamamelis. 
Its close structural resemblance to C. ulmicola and H. hamamelis 
and unlikeness to P. p.-transversus and P. p.-caulis is due to the 
fact that P. vagabundus, C. ulmicola, and H. hamamelis are 
formed on the blades of the leaves, while P. p.-transversus and 
P. p.-caulis are formed on the petioles which are made up largely 
of fibro-vascular tissue. My specimens of these galls were mature, 
and I am therefore unable to say anything concerning their early 
stages. 

In the Phylloxera galls all the cells are at first rich in pro- 
toplasm and the tannin does not form in abundance until very 
late. The two zones are. fairly prominent. In P. c.-caulus Fitch 
on H. ovata, a gall which forms on both blade and petiole of the 
leaf and also on young stems large intercellular spaces are formed 
near the surface. 

3. GALLS OF f-SVLLIDAE. 

Pachypsylla c. -mamma Riley has been described in Part V 
(Figs. 59, 60). 



April, 1904.] Galls and Insects Producing Them. 119 



4. GALLS OF CECIDOMYIA. 

These galls have been described in Part I (Figs. 22, 23, 24), 
in Part V (Figs. 61, 62, 63), in Part VI (Figs. 73, 74, 75), and 
in the Appendix (Figs. 114-119). In these galls the two zones 
are usually fairly well defined, but the galls of this genus are so 
different in character that it is difficult to give a definite descrip- 
tion. The time for the formation of the tannin is variable, but 
it is usually produced late and in great abundance. 

5. GALLS OF THE CYNIPIDAE. 

All these galls are very similar. The majority show the four 
zones and in most cases these zones are well defined. The outer 
zone is the epidermal which will be described later (Figs. 84-91). 
The second is the parenchyma zone ; the third is the protective 
zone made up largely of sclerenchyma, and the fourth or inner- 
most is the nutritive zone. In many cases the second and third 
zones become partially or entirely separated. This separation, 
however, is not between the second and third zones as previously 
stated by me in Parts I and V, and by Fockeu, but rather a sep- 
aration of the tissues of the second or parenchyma zone, the 
greater part of this zone clinging to the epidermal zone and a few 
cells remaining attached to the protective zone. 

Diastrophus siminis Bassett (Figs. 66-69) has been described 
in Part V. The four zones are distinct and each shows the char- 
acter previously referred to. 

Diastrophus nebulosus O. S., described in the Appendix (Figs. 
129a, b), is a stem gall in which the zones are well defined, the 
protective zone being especially well developed. Each zone shows 
the characters previously referred to. 

In Amphibolips confluentus Harris (Figs. 121a, b, c) the first 
and second zones are well developed, but the distinction between 
the third and fourth is not so pronounced. 

In Amphibolips inanis O. S. (Fig. 28) the four zones are well 
defined. In the young gall (Fig. 79) the cells of the nutritive 
zones are very rich in protoplasm and there is very little or no 
distinction between the nutritive and the protective zone, but as 
the galls approach maturity the cells of the protective zone 
become very thick and are soon converted into sclerenchyma 
(Fig. 80). 

In Callirhytis papillatus O. S. we have the four zones well 
defined (Fig. 30). As the gall approaches maturity the cells of 
the nutritive zone lose their protoplasmic contents and become 
very much shriveled, the protective zone is made up usually of 
only two or three layers of cells. Next to the protective zone are 
two or three layers of cells which are in reality a part of the 
parenchyma zone. The large intercellular spacts formed in this 



i2o The Ohio Naturalist. [Vol. IV, No. 6, 

zone are bridged by long unicellular threads, but no fibro-vascular 
bundles (Fig. Si) 

Dryophanta palustris O. S. galls show the four zones well 
defined (Figs. 29, 65). When mature the contents of the cells 
of the nutritive zone has been entirely used by the insect. The 
protective zone consists of only two or three layers of sclerenchyma 
cells, to which are attached a few cells of the parenchyma zone 
(Fig. 82). 

Andricus petiolicola Bassett (Fig. 124) produce a very hard 
petiole or mid-rib gall which shows the four zones well defined. 
There is no separation between the second and third zones. The 
nutritive zone is at first very prominent, but it is reduced as the 
gall approaches maturity. The protective zone developes its 
sclerenchyma character rather late (Fig. 83) and gradually merges 
into the two adjacent zones. 

B. Epidermal Structures. 

The epidermal cells vary in the size and in the thickness of the 
cell walls. The galls may be smooth, pubescent or covered with 
spiny structures. The amount of pubescence depends somewhat 
on the natural pubesence of the host plant. Galls on such smooth 
plants as Populus deltoides Marsh show very few and very small 
trichomes, while galls on plants that are naturally pubescent are 
likely to be pubescent. These trichomes vary in shape and gen- 
eral character and are very prominent when the gall is young. 
As the gall approaches maturity the trichomes usually disappear. 
When these trichomes drop off their place of former attachment 
is marked by a small mass of small cells, usually containing 
tannin and from which imperfect rows of cells seem to radiate 
(Figs. 84-90). 

I. GALLS OF CYNIPIDAE. 

Dryophanta palustris O. S. is very pubescent when young 
(Fig. 84a). In the mature gall the cells are much larger, the 
trichomes have disappeared and their point of attachment is made 
visible by the accumulation of tannin (Fig. 84b). 

All my specimens of Amphibolips inanis O. S. were fully 
developed, but the points where the trichomes had evidently been 
attached were very prominent (Fig. 85). These points are the 
large, black spots so prominent on these large bladdery galls. 

In Diastrophus siminis Bassett the trichomes are very large 
(Fig. 86") and drop off very readily. 

In Diastrophus potentillae Bassett the trichomes are very 
numerous and each is at the apex of a very small elevation (Fig. 
87). Examination of the epidermis of Acraspis eriuacei Walsh 
show that its spines were due to similar but much more prominent 
elevations. 



April, 1904.] Galls and Insects Producing Them. 



2. GALLS OF THE APHIDIDAE. 

Galls belonging to this family are usually less pubescent than 
those belonging to the Cynipidae. The triehomes are usually 
much shorter and frequently less numerous. Each trichome is 
usually made up of a single cell (Fig. 88). The place where 
these triehomes were attached is marked by an accumulation of 
tannin, the same as in the Cynipidous galls (Figs. 89, 90). 

Examination of the galls of the Phylloxera spinosa Shinier 
show that the spines were due to the same cause as in the 
Cynipidous galls (Fig. 87). 

Galls of Pemphigus p.-transversus Riley (Fig. 91) and P. p.- 
caulis Fitch were perfectly smooth, but the cell walls were much 
thicker than in any other galls studied. 

conclusion. 

1. The inner layer of cells (i. e., those next to the larva) are 
always supplied with nutriment until the insect is mature. 

2. The development of the other layers of cells is for the pro- 
tection of the larvae. These protective devices reach their highest 
development in the Cynipidous galls. 

3. In the very young galls there is usually little or no distinc- 
tion between the nutritive and protective zones. The time of the 
differentiation of the protective zones varies in different species. 

4. The fibro- vascular bundles are most prominent in galls on 
the petiole and mid-rib. 

5. Most galls are covered with triehomes which disappear as 
the galls approach maturity. The number of triehomes is varia- 
ble in proportion to the pubescence of the host plant. 

6. Spines are due to elevations composed almost entirely of 
epidermal cells. 

Part IX. Ovipositors and Mouthparts. 

One of the most prominent questions concerning the formation 
of galls which presents itself to the students of entomology and 
botany and even to the most casual observer, is the exciting factor 
in gall production. Is the stimulus from the ovipositor or mouth- 
parts ? Is it mechanical or chemical ? The author believing that 
the logical method of solving this problem was to first make a 
careful study of the morphology and development of galls has 
published the preceding parts of this paper. The author does 
not claim to have found a complete solution of the problem, but 
is hopeful that some of the facts stated in this series of papers 
ma}- lead to more thorough and satisfactory studies of the prob- 
lem. The problem presents many difficulties ; the parasites and 
inquiliues which are usually present are frequently difficult to 
distinguish from the real gall- maker ; this is especially true when 
the study is confined to the larvae. In the following studies the 
author is reasonably certain that the determinations are correct. 



The Ohio Naturalist. [Vol. IV, No. 6, 



OVIPOSITORS. 

Gall-making insects deposit their eggs by two methods, either 
on the surface of the plant or within the tissues. Those insects 
which deposit their eggs on the surface usually have mouthparts 
developed for sucking, while those which deposit their eggs 
within the tissues usually have mouthparts developed for biting. 
Those which deposit their eggs on the surface of the plant are 
the Acarina, the Hemiptera, and the Diptera. Those which 
deposit their eggs within the tissues are the Hymenoptera and the 
Lepidoptera. In this paper we have made a careful study of the 

ovipositors of Cecidomyia gleditsiae, of Nematus sp — , Dry- 

ophanta palustris, Amphibolips radicola, Andricus cornigerous, 
A. seminator, and Rhodites radicum. A number of others were 
examined, but because of the uncertainty as to determination are 
not figured. 

The Cecidomyia ovipositor (Fig. 92) is not suited to punctur- 
ing tissues. The gall is never formed until after the hatching of 
the larva. In this case it is evident that the stimulus, whether 
mechanical or chemical, is produced by the larva. 

Insects belonging to the genus Nematus deposit their eggs 
either on the surface of the plant or in slits made by the ovipositor 
(Figs. 93a, b). It is said that the galls are formed from these 
wounds before the larva escapes from the egg, and in these cases 
it is claimed that the irritating cause is a drop of fluid secreted 
by the parent insect. Westwood claims that the egg increasing 
in size is a result of imbibing sap from the wound in the plant. 
It is well known that the eggs of some insects increase in size as 
a result of the growth of the embryo within the egg. I have so 
far been unable to make any satisfactory observations upon the 
Nematus galls, but it is probable that the eggs increase in size 
from the growth of the embryos and not as a result of the absorp- 
tion of plant sap. It is also possible that the gall may be the 
result of the mechanical irritation of the ovipositor or the enlarge- 
ment of the egg or both. The wound caused by the ovipositor 
of the Nematus is very much more severe than the wounds caused 
by the ovipositors of the Cynipidous insects. 

Adler, after a careful observation on Nematus Vallisnierii, says: 
"This fly, which is armed with a finely serrated terebra, cuts 
into the tender leaves of the end of the shoot of the Salix amyg- 
dalina, and inserts her egg into the open wound, frequently plac- 
ing several in the same leaf. At the same time the glandular 
secretion flows into the wounded leaf. A few hours after this 
injury the leaf surface presents an altered appearance, and new 7 
cell formation begins freely, leading to a thickening of the sur- 
rounding leaf surface. After the lapse of about fourteen days 
the green and red-shaped gall is fully grown. If it be now 



April, 1904.] Galls and Insects Producing Them. 123 

opened the egg can still be seen lying within the cavity. The 
embryonic development is as yet unfinished and three weeks 
elapse before the larva emerges from the egg to find around it the 
material prepared for its nutriment. In this case the wound 
caused by the fly is the immediate exciting cause of cell activity, 
and leads to gall formation." 

M. W. Beyerinck, in a paper regarding the growth of the gall 
of Nematus caprea on Salix amygdalina holds a similar view. I 
have not seen this paper, but an abstract* of it says: "The 
production of the gall is undoubtedly due to the matter secreted 
by the poison gland, which is, consequently, homologous with 
the poison of Hymenoptera aculeata ; when the insect does not 
deposit an egg in the wound which it makes, the quantity of 
albuminous matter poured into the vesicle is always less than 
when an egg is deposited ; by careful observation it is possible to 
assure oneself that the size of the gall is always proportional to 
the size of the wound and the quantity of albuminoid matter 
introduced. By an experiment in which a deposited egg was 
punctured by a fine needle, it was shown that the gall is due to 
the parent and not to the egg ; but, of course, in such a case the 
gall remains small ; neither the egg nor the larva are necessary 
for its production, though their presence exercises a certain influ- 
ence on the regularity of their development." 

The ovipositors of the Cynipidae vary in length and in the 
amount of coiling within the abdomen. All present the same 
general characters. So far I have been unable to detect any 
relationship between the length and character of the ovipositors 
and the location and complexity of the galls (Figs. 94 to 98). 
Adler claims that the egg is always deposited in or near the 
Cambium layer of the plant. I am inclined to accept this state- 
ment, but have made no special effort to verify it. If Adler' s 
observations are correct the length of the ovipositor would be 
associated not with the depth of the Cambium from the surface 
of that part of the mature plant affected, but with the location 
of the Cambium at the time of oviposition and with the difficul- 
ties which the insect would experience in forcing the ovipositor 
to the desired point. 

Oviposition usually occurs before the buds are open, and the 
eggs may be placed in three positions (1) in the stem, as in the 
case of Rhodites radicum O. S., R. globulus Beut., Andricus 
cornigerous O. S. ; (2) in the apex of the incipient stem as in 
Andricus clavula Bassett, and Holeaspis globulus Fitch ; or (3) 
in the leaves of the bud as in Rhodites bicolor Harris, Amphi- 
bolips confluentus Harris, A. inanis O. S., A. ilicifoliae Bassett, 
Neuroterus irregularis O. S., A. seminator, Callirhytis tumifica 



'Jour. Roy. Micr. Soc, 1887, p. 746. 



124 The Ohio Naturalist. [Vol. IV, No. 6, 

O. S., Holcaspis centricola O. S., Dryophanta palustris O. S., 
and Callirhytis papillatus O. S. In these cases it is evident that 
the force necessary to penetrate the bud may be as great or even 
greater than the force necessary to penetrate a stem. Adler's 
observations demonstrate that great force is used to penetrate the 
buds and reach the desired point for depositing the eggs. 

Beyerinck has demonstrated that the fluid ejected by the ovi- 
positor of the Cynipidae is very different from the fluid ejected 
from other Hymenopterous insects ; that it is without taste or 
smell and does not irritate when injected under the skin. Adler 
has demonstrated that this fluid cannot be considered as the stim- 
ulus to gall production. It is probable that it may serve to attach 
the eggs, or as an antiseptic, or as a seal for the wound. 

Since the gall does not form until after the hatching of the 
larva it is evident that oviposition does not furnish the stimulus 
unless it may be that there is cell division but no swelling of the 
plant tissues previous to the hatching of the larva. The author 
has made no observations upon this point. Adler, in discussing 
this question, says, in regard to Trigonaspis : " This fly pricks 
the leaf in May, but months pass before any trace of gall forma- 
tion can be seen. It has tolerably strong ovipositor with which 
it cuts into the veins of the leaf, and in this way a distinct mark 
is left wherever an egg has been inserted. Guided by these 
marks it is easy to find the egg, but it is not until September that 
the larva leaves the egg, and then gall formation begins." 

MOUTH PARTS. 

Since oviposition does not give an explanation of the stimulus 
causing the formation of the gall it is necessar}' for us to turn our 
attention to the mouthparts. 

For convenience the insects may now be divided into two 
groups, those with mouthparts for sucking, which make their 
attacks upon the outside, and those with mouthparts for biting, 
which make their attacks from the inside. Under the former are 
included the Acarina, the Hemiptera and the Diptera ; under the 
latter are included the L,epidoptera and the Hymenoptera. 

I. HEMIPTERA. 

The Hemipterous insects which produce galls may be placed in 
the following order, with reference to the complexity of their 
galls, beginning with the lowest : Schizoneura, Colopha, Horma- 
phis, Phylloxera, Pemphigus and Pachypsylla. Mouthparts of 
the following were carefully examined : Schizoneura americana 
Riley, Colopha ulmicola Fitch (Fig. 99), Hormaphis hamamelis 
Fitch, Phylloxera carya-fallax Riley, P. c.-globuli Walsh, P. 
c.-spinosa Shinier, P. vastatrix Planchon, Pemphigus populi- 
transversus Riley, P. p.-caulis Fitch, P. vagabundus Walsh, 



April, 1904.] Galls and Insects Producing Them. 125 

Pachypsyllaceltidis mamma Rile)' (Figs. 100a, b), and P. c. -gemma 
Riley. 

The study of these mouthparts gave no new anatomical facts. 
The different genera showed considerable variation as to length 
of beak and setae. In general it may be said that the setae tend 
to increase in the distance they may be protruded beyond the tip 
of the beak as the galls approach complexity. This, however, 
cannot be considered an exact rule, since the S. americana, C. 
ulmicola and H. hamamelis have setae of practically the same 
length, although the gall produced by S. americana is much 
simpler than the galls produced by either C. ulmicola and H. 
hamamelis (Part I, Figs. 12, 13 and 15). It was impossible to 
make exact measurements of the distance the setae protruded 
be3^ond the tip of the beak, since it was impossible to tell whether 
the setae were full}' extended or partial^ retracted. The above 
conclusions were reached after the examination of a large number 
of specimens. 

So far as I have been able to determine the insects do not 
remain attached to any one point for a great length of time. The 
P. c. -mamma (Figs. 100a, b) has a gall of the greatest complex- 
ity, and the insect has setae which protrude farther beyond the 
point of the beak than any other examined ; a large number of 
these galls were opened and the position of the insect noted. The 
insect was never found attached and apparently had no definite 
point of attack. 

The preceding observations emphasize Conclusions 6 and 8 of 
Part I and a statement in the first of Part V. That is, the modi- 
fication of the plant tissue to form the gall is purely mechanical, 
being a continuous effort on the part of the plant to heal the 
wound produced by the repeated puncturing of the cells by the 
insect. When a branch is cut from a tree a growth is produced 
which tends to cover the wound. In this case a single wound 
and a single stimulus which is purely mechanical but which pro- 
duces rapid growth for the purpose of covering the wound. In 
the case of Aphididae and the Psyllidae galls the wounds are 
more slight but repeated rapidly, the stimulus is mechanical and 
the growth rapid, tending to cover the injury. 

It is possible that the setae of the various genera may stimulate 
different tissues and thus cause galls of varying complexity, but 
upon this question I am not ready to give a definite statement. 

2. DIPTERA. 

The Cecidomyid galls occur upon a greater variety of hosts than 
any other group of galls, and as previously stated in Part V, show 
by far the greatest variation in structural characters and the 
smallest number of typical characters. 



126 The Ohio Naturalist. [Vol. IV, No. 6, 

The mouthparts of a number of larvae were examined (Figs. 
101, 102), and all were practically the same; salivary or other 
gland structures could not be demonstrated. 

I am inclined to believe that the Cecidomyid galls are due to 
purely mechanical stimuli and that the great variations are due 
to the different tissues upon which the larvae feed. 

Mr. W. A. Cannon,* in discussing a Cecidomyid gall on the 
Monterey pine, says that the "larvae take their food only by 
absorption through the surface of the body," also that " there is 
no indication that the hypertrophy is either caused or affected by 
any substance deposited with the eggs." 

3. HYMENOPTERA. 

We now come to the galls of greatest complexity and also to 
those with which we have the greatest difficulty. These galls are 
so very generally infested with parasites and inquilines that it is 
difficult to decide which larva is the true gall producer. 

A careful study of these shows that the insects have a very 
strong pair of mandibles (Figs. 103 to 108), each working upon 
two pivotal points. Some of these mandibles appear to have an 
opening at the tip (Figs. 104, 105), and some showed what 
appeared to be sacs or glands at the base (Figs. 104, 106b). In 
one case at least (Fig. 104) these glandular sacs appeared to be 
connected with the opening. The question that naturally pre- 
sents itself is, are these openings for the purpose of pouring out 
a fluid or are they suctorial as in the case of Chrysopa and other 
families? In only two species was it possible to demonstrate 
these structures. Some light is thrown upon this by Part VIII, 
in which it was shown that the cell walls of the inner or nutritive 
zones were not destroyed, but that the contents of the cells were 
removed, causing them to shrivel. 

The teeth of the mandibles are never on the same plane and 
the mandibles become more and more chitinous as the larvae 
approach maturity. The strength of the mandibles appears to 
depend upon the density of the tissue through which the insect 
works its way to the outside In A. inanis (104) and A. con- 
fluentus (Fig. 105) the strength of the mandibles is practically 
the same and the character of the galls very similar. In D. sim- 
inis (Figs. 106a, b) the mandibles are stronger and the tissues of 
the gall correspondingly denser. C. petiolicola (Fig. 103) is by 
far the strongest of those studied, and the tissues through which 
the insect must work its way the densest of the leaf galls (Fig. 
124). 

A study was made of the larvae from galls of C. papillatus. 
This is a small, rather dense leaf gall. Farvae of two species 

"Cannon, W. A. "The Gall of the Monterey Pine." The American Naturalist, Vol. 
XXXIV, No. 406 (Oct., 1900), p. Soi. 



April, 1904.] Galls and Insects Producing Them. 1 2 7 

were found (Figs. 107, 108). A careful study of the mouth- 
parts lead me to consider No. 107 as a true gallmaker and No. 
108 as a parasite. The mouthparts of the one which I consider 
a true gallmaker were as strong as those of C. petiolicola (Fig. 
103). The mandibles of the parasite (108) were equally strong 
and showed what appeared to be rudimentary gland structures. 

Holcaspis globulus Fitch was the only bud (i. e., incipient 
stem gall, Part III, Fig. 34) gall examined. In the young larvae 
the mouthparts are weak, but as the larvae approach maturity 
the mandibles become very strong (Fig. 109) and well fitted to 
cut the opening for the escape of the insect. However, the 
mouthparts were not so strong as in the case of C. petiolicola, but 
the gall of H. globulus is not so dense as the gall of C. petiolicola. 

The mouthparts of Nematus poraum Walsh (Fig. no) were 
very similar to those of the Cynipidae. I am not inclined to con- 
sider the apparently glandular-like structure observed in a few 
species of any great importance. They may be suctorial or they 
may be degenerate organs. I consider the stimulus as purely 
mechanical. The character of the gall may depend upon the 
location, which would result in difference in tension in different 
parts of the plant on which the gall may be located and also upon 
the laws of natural selection, which will be considered in the latter 
part of this paper. 

It would be interesting to know the exact time that cell divi- 
sion begins in the formation of a gall, but it is very difficult to 
make satisfactory observations upon this point. Adler has made 
successful observations upon this stage in Neuroterus laviusculus 
and Biorhiza aptera. He says: "The moment the larva has 
broken through the egg covering and has for the first time 
wounded the surrounding cells with its delicate mandibles, a 
rapid growth begins. This goes on so quickly that while the 
posterior part of the larva is still within the covering a wall of 
like growth of cells has already arisen in front. This rapid cell 
increase can be easily explained because the irritation set up by 
the emerging larva is exerted upon highly formative cells which 
collectively possess every condition of growth. The cells which 
are primarily around the larva cannot be distinguished from the 
parenchymatous cells from which they proceed." 

4. LEPIDOPTKRA. 

A careful study was made of the mouthparts of the Gelechia 
solidaginis Fitch (Fig. in) and upon an undetermined species 
found upon Rudbeckia laciniata (Part VI). The mandibles are 
larger and much stronger than in any of the Hymenopterous 
galhnakers which I examined. The gall is also much stronger 
than any of the Hymenopterous galls whose larvae were studied. 
No glandular structures were observed. 



i28 The Ohio Naturalist. [Vol. IV, No. 6, 



CONCLUSION. 

i. The fluid secreted by the ovipositor is not an irritant, and 
therefore cannot be the stimulus for gall production. 

2. Since the gall does not form, excepting the Nematus galls, 
until the appearance of the larvae, it is improbable if oviposition 
is a stimulus for gall production ; and in those insects in which 
the egg is not deposited within the tissues of the plant it is 
impossible. 

3. Glandular structures were observed in only a few of the 
Hymenopterous larvae and these were of doubtful character. 

4. Since it has so far been impossible to demonstrate the 
presence of a chemical stimulus except in Nematus, we must 
consider that the stimulus is usually mechanical. As previously 
stated (Part I, Conclusion 3) the morphological characters of the 
gall depend upon the genus of the insect producing it rather than 
upon the plant upon which it is produced. The early history of 
all galls except the Cecidomyid is practically the same ( Part V, 
Con. 2). The shape and external character of the gall probably 
depends upon the following : ( 1 ) The plant upon which the 
attack is made; (2) Upon the part upon which the attack is 
made ; (3) Upon the tissues affected ; (4) Upon possible results 
of natural selection. 

SUMMARY OF PARTS. 

Next in importance to the problem of a stimulus giving rise to 
a gall is the explanation of specific external characters. This 
question is not easily answered and at the present time any 
explanation must be largely theoretical. 

The gall-producing insects are found in six orders, as follows : 
1. Arachnida (mites); 2. Hemiptera (Aphidae and Psyllidae); 
3. Diptera (Cecidomyidae and Trypetidae); 4. Hymenoptera 
(Cynipidae and Tenthrenidae); 5. Uepidoptera, and 6, Coleop- 
tera. The gall-producing habit must have originated independ- 
ently in each of these orders and in some orders (Diptera and 
Hymenoptera) it must have originated independently in each of 
the two families represented. 

The formation of the gall is due to two primary factors ; a 
' stimulus, usually mechanical, given by the insect, and nourish- 
1 ment furnished by the plant. 

Conclusions reached as results of previous studies and bearing 
on this subject are as follows : 

1. "Galls maybe classified into two general groups, viz.: 
those produced by mouthparts and those produced by oviposition. 
Those produced by oviposition may be considered the more highly 
developed." (Part I, Con. 1.) 



April, 1904.] Galls and Insects Producing Them. 129 

2. " The gall does not form until the appearance of the larvae. 
Therefore all galls are produced by mouthparts." (Part VIII, 
Con. 1.) The Nematus galls are an exception. 

3. "The morphological character of the gall depends upon 
the genus of the insect producing it rather than upon the plant 
on which it is produced." (Part I, Con. 3.) 

4. ' ' Within each family we find certain morphological resem- 
blances." (Part I, Con. 4.) 

5. "The families show parallel lines of development from a 
low form of gall structure up to a high form." (Part I, Con. 5. ) 

6. " The presence of at least two zones, of which the inner 
may be considered nutritive." (Part I, Con. 7. ) 

7. "The formation of the gall is probably an effort on the 
part of the plant to protect itself from an injury which is not 
sufficient to cause death. Both Adler and Fockeu consider that 
after the first stages of formation the gall becomes an independ- 
ent organism growing upon the host plant. This is probably true 
in the highly developed galls of Aphididae, Cecidomyia, and 
Cynipidae, but the writer is doubtful if this is true in the less 
complex galls of Acarina, Aphididae and Cecidomyia." (Part I, 
Con. 8 and Part V, Con. 6.) 

8. "In the formation of all leaf galls except the Cecidomyia 
galls the normal cell structure of the leaf is first modified by the 
formation of a large number of small, compact, irregularly shaped 
cells. In the galls of Acarina and Aphididae this is followed by 
a development of trichomes, especially in the former. In all 
galls the mesophyll is subject to the greatest modification. Many 
small fibro-vascular bundles are formed in this modified meso- 
phyll." (Part V, Con. 2.) 

9. "Trichomes are far more common in galls produced by 
mouthparts than in those produced by oviposition." (Part V, 
Con. 9, and see Summary 2.) 

10. "Variation in galls is due to their being produced by 
insects of different orders, to their working upon different parts 
of the plant and upon different tissues of these parts." (Part 
III, Con., and Part IV, Con. 1.) 

I. ARACHNIDA. 

The Arachnida galls are of four types : (1) A modification in 
the epidermis of the leaf as in the Phytoptus galls on maple and 
elm ; (2) A fold in the plant tissue causing a cavity filled with 
trichomes. among which the parasites live, as in the case of many 
Phytoptidi (Figs. 8, 9, 10, 11, 43, 44, 45, Parts I and V) ; 
(3) A swelling with an exposed surface covered with trichomes, 
among which the parasites live, as in the case of Erineum 



i3° The Ohio Naturalist. [Vol. IV, No. 6, 

anomalum (Part V, Figs. 47, 48); (4) The witehbroom forma- 
tion, as in the case of the Phytoptus sp , and Sphaerotheca 

phytoptophila Kell. and Sw. on Celtis occidentalis. 

The author has studied only the second and third types. The 
difference between these two may be accounted for by the fact that 
the Phytoptus attacks the blade while the Erineum attacks the 
petiole, mid-rib or larger vein. The part affected undergoes a 
curvature in each case in the direction of the least resistance. 

2. HEMIPTERA. 

The method of attack by the Hemiptera is practically the same 
as in Arachnida, i. e., by sucking mouthparts. The galls present 
a complete serial line of development, the lowest form being a 
simple curling of the leaf as in the case of Schizoneura americana, 
the next higher, a simple folding of the leaf, as in the case of 
Colopha ulmicola, the next higher is a more complex structure, 
such as the Phylloxera galls and H. hamamelis, the next higher, 
the slightly more complex, as in the case of the Pemphigus galls 
(Figs. 12 to 21, and 49 to 58). The galls of the Pachypsylla 
(Figs. 59, 60) are the most highl) 7 developed of the entire series. 

Although in this case we have a complete series, it is difficult 
to understand how this development has been produced. It may 
be that the different forms are due to the attack being made upon 
different tissues in each case, or to the degree in which the tissues 
are injured. Upon this point we have no direct proof. However, 
there is very little doubt that the stimulus is entirely mechanical. 

3. DIPTERA. 

As previously stated, the Cecidomyid galls are far more varied 
in location and in morphological structure than any other group 
of galls and show less number of characters peculiar to them- 
selves alone. There is not sufficient data to draw even theoretical 
conclusions concerning the influencing causes in their devel- 
opment. 

4. HYMENOPTERA. 

As previously stated, the Cynipidous galls are the most highly 
developed and show a greater number of morphological structures 
peculiar to themselves than any other group (Part I, Con. 2 ; 
Part V, Con 3). 

Since the gall does not begin to develop until after the hatching 
of the larvae, oviposition cannot be an important factor except in 
so far as it is necessary to have the egg placed in certain tissues. 

Examination of the mouthparts show few, small and insignifi- 
cant gland-like structures the character of which is doubtful. It 
is therefore probable that the stimulus is purely mechanical except 
in the Nematus. But how are we to account for the s:reat mini- 



April, 1904.] Galls and Bisects Producing Them. 1 3 1 

ber of specific external characters ? Fet us first review the struc- 
tural characters of the leaf galls, since these galls show the most 
uniform line of development. Considering Neuroterous irregu- 
laris the gall of greatest simplicity, we can formulate the following 
diagram : 

— C. papillatus. 

— A. confluentus. 
-. — H. centricola. 
- — A. inanis. 



N. irregularis — C. tumifica. 



-D. palustris. 
-A. petiolicola. 



Iii N. irregularis the zones are not so well developed as in 
C. tumifica. In C. tumifica the zones are perfect, but in contact. 
In C. papillatus the protective and parenchyma zones are sepa- 
rated, but connected by long parenchyma cells. In H. centricola 
and A. inanis the protective and parenchyma zones are connected 
by fibro-vascular bundles. In A. confluentus they are connected 
both by fibro-vascular bundles and by parenchyma cells (Fig. 
121). In D. palustris the parenchyma and protective zones are 
not connected. In A. petiolicola the zones are in contact, but 
the tissues are very dense, due to location in the petiole of mid- 
rib of the leaf. 

If galls become independent structures they are undoubtedly 
subject to the same laws of natural selection as any other group 
of organisms, or if they be considered as parts of the plant they 
must also be subject to the same laws of natural selection as any 
other part of the plant on which they live. How, then, have 
these laws affected the gall ? It may be a protective coloration 
against birds and rodents, and other insects, but this cannot be 
very important since man}- species of galls are very conspicuous. 
Furthermore, animals make but very little use of galls for food. 
So far I have observed other animals using galls for food but 
once and then birds were tearing open the large galls of Pemphi- 
gus vagabundus and eating the insects. The tannin which devel- 
ops in such abundance in all galls as they approach maturity is 
probably a great protection against insectivorous animals. 

The greatest insect enemy with which the gall insect has to 
contend is the great number of parasites. The size, shape and 
character of the epidermal covering of the gall may be a protec- 
tion against this numerous enemy. The thickness of the gall and 
the density of the tissues, especially the protective zone, is an 



132 The Ohio Naturalist. [Vol. IV, No. 6, 

important protective device. The large intercellular chambers in 
the parenchyma zone place the larvae at a great distance from the 
surface of the gall without increasing the amount of work neces- 
sary for the mature insects to accomplish before reaching the 
outside ; this is undoubtedly a great protection against parasites, 
since it increases the difficulties for the parasite in reaching the 
larvae with the ovipositor, The development of these protective 
devices is probably the result of natural selection. Since the 
character of the gall depends upon the insect, many variations in 
the gall may also depend on variations in the stimuli given by the 
insect. If these variations in character of epidermis, in thickness 
of parenchyma zone, in the formation of large intercellular spaces, 
in thickness and density of protective zone, are advantageous to 
the insect in protecting it from the numerous parasites, these 
characters may be perpetuated in succeeding generations and the 
gall may increase in complexity. Natural selection is a reasona- 
ble explanation. 

It should be remembered that the plant is making an effort to 
resist a parasite from which it cannot escape. The gall-maker 
derives its nourishment without destroying its host and at the 
same time strives to protect itself as far as possible from the great 
number of parasitic enemies. The food supply first becomes a 
part of the gall and upon this supply which, in the case of the 
Cynipidae, is stored in the nutritive zone, it feeds. 

Any irritation, such as the cutting or puncturing of plant tis- 
sues, may and usually does cause excessive growth. It is proba- 
ble that the primitive galls were of a type similar to the simplest 
of the Phytoptus galls, i. e., a peculiar growth of the epidermal 
cells. The next step in the evolution of the gall may be repre- 
sented by a type similar to Schizoneura americana, in which case 
the stimulus is greater, resulting in a curling of the leaf. The 
next step may be represented by a type similar to the more com- 
plex Phytoptus galls, H. hamamelis, C. ulmicola, the Phylloxera, 
the Pemphigus and the most complex of the Pachypsylla galls in 
which we find a series of more or less complex folds in the leaf up 
to the increase in amount and differentiation of the tissue as in 
the case of P. p. -mamma. 

In the Cynipidous galls we have the greatest complexity, but 
also a factor somewhat different from that in the forms to which 
we have referred, i. e., the placing of the egg below the surface 
and in those tissues upon which the larva is expected to feed. 
It is impossible to say whether this habit of placing the egg below 
the surface was acquired before or after the gall-making habit, 
but it must be a great advantage to the insect. These galls, as 
previously demonstrated, show the more complex serial line of 
development of any of the galls, but even the simplest of these is 
more complex than the most complex gall produced by any other 



April, 1904.] Galls and Insects Producing Them. 133 

order of insect. This very complex development is due to an 
early acquirement of the gall-making habit or to more rapid evo- 
lutionary development as a result of the deposition of the egg 
below the surface. 

The greater part of the work connected with Part IX of this 
series was conducted at the Eake Laboratory of the Ohio State 
University at Sandusky, Ohio, and I am very much indebted to 
the Director, Professor Herbert Osborn, for valuable assistance. 
I also wish to express my thanks to the many friends who have 
collected material and otherwise aided in these studies. 

This series of papers will be presented to the Faculty of the College of 
Arts, Philosophy and Science, of the Ohio State University, as the thesis 
requirement for the degree of Doctor of Philosophy, June, 1904. 

LITERATURE. 
Continuous with the bibliography published with Parts I and II. 

24. Adler, Dr. H. " Eege-Apparat und Eierlegen der Gal 1- 
wespen." Deutsche Entomologische Zeitschrift XXI. 1877, 
Heft II. 

25. Beyerinck, Dr. M. W. " Beobachtungen uber die ersten 
entwicklungsphasen einiger Cynipideugallen." Veroffentlicht 
durch die Konigliche Akademie der Wissenschaften zu Amster- 
dam, 1882. 

26. Beijerinck, M. W. " Bydrage tot de Morphologie der 
Plantegallen," 1877 

27. Beijerinck, M. W. " De Gal van Cecidomyia aan Poa 
nemoralis," Overgedrukt uit het Maandblad voor Natuurwet- 
enschatten, 1884, No. 5. 

28. Beijerinck, M. W. " Ueber Gallbildung und Genera- 
tionswechsel bei Cynips calicis und uber die Circulansgalle," 
Verhandelingen der Koninklijke Akademie van Wetenschappen 
te Amsterdam, 1894. 

29. Beijerinck, M. W. " Sur la Cecidiogenese et la Genera- 
tion Alternante chez le Cynips calicis. Observations sur la galle 
deL/Andricus circulans," Extrait des Archives Neerlandaises, 
T. XXX, p. 387-444- 

30. Busgeu, M. "Der Honigtau Biologische Studien an 
Pflanzen und Pflanzenlausen. ' ' Zeitschrift fur Naturwissenschaft. 
Bd. XXV. 

31. Busgen, M. " Zur Biologie der Galle von Hormomyia 
Fagi Htg. " Forstlich-naturwissenschaftliche Zeitschrift, Jan- 
uar, 1895. 

32. Courchet, M. L. " Etude sur less Galles Causees par des 
Aphidiens." Akademie des Sciences et Eettres de Montpellier. 
Memoires de la section des sciences. 

33. Derbes, M. " Observations sur les Aphidiens qui fontles 
Galles des Pistachiers." 



134 The Ohio Naturalist [Vol. IV, No. 6, 

34. Eckstein, Dr. Karl. " Pflanzengallen und Gallenthiere. " 

35. Fockeu, H. " Etude sur Quelques Galles. " Paris, 1897. 

36. Heim, Dr. F. " Observations sur les Galles produites 
sur Salix babylonica par Nematus salicis." 1893. 

37. Kessler, Dr. H. F. " Die Entwicklungs und Eebens- 
geschichte der Gallwespe Cynips calicis Brgst. und der von 
derselben an den weiblichen Bluthen von Quercus pedunculata 
Ehrh. horvorgerufenen Gallen, Knoppern genannt." Cassel, 
1897. 

38. Eacaze-Duthiers, M. Recherches pour servir a E'His- 
toire des Galles." Extrait des Annals des Sciences Naturelles, 
Tome XXIX. 

39. Molliard, Marin. ' ' Recherches sur les Cecidies Florales." 

1895. 

40. Nabias, Dr. B. " Ees Galles et leurs Habitants." Paris, 

1886. 

41. Tschirch, A. " Ueber durch Astegopteryx, eine neue 
Aphidengattung, erzeugte Zoo-cecidien auf Styrax Benzoin 
Dryan." Deutschen Botanischen Gesell-schaft. 1890, Band 
VIII, Heft 2. 

42. Rubsaamen, Ew. H. " Uber Zoocecidien von der Balkan- 
Halbinsel." 

43. Paszlavszky, Jozsef. "A Rozagubacks Fejlodeserol." 
Termeszetrajzi Fuzetek, Vol. V, Parte II-IV. Budapest, 1882. 

44. Paszlavszky, Jozsef. " Beitrage zur Biologie der Cynipi- 
den." Wiener Entomologische Zeitung II. 1883, Heft 6. 

45. Prilleieux, M. Ed. " Etude sur la Formation et le Devel- 
opment de Quelques Galles." 

46. Pierre, M. l'abbe. " Ea Mercuriale et ses Galles." 
Extrait de la Revue scientifique du Bourbonnais et du Centre de 
la France, Juin, 1897. 

EXPLANATION OF PLATES IX-XII. 

The drawings were made with a Bausch & Lomb microscope. For Figs. 
70-76 and Figs. 84-91 and Fig. 93b, a Number 2 ocular and ye objective. 
For Figs. 77-83, a Number 2 ocular and ^ immersion objective. With 
Figs. 92-98 and Figs. 106a, no and in, a }± ocular and 2 ; objective. For 
Fig. 93 a Number 2 ocular and %' objective. The reduction is not so great 
as in the preceding parts and therefore the figures are proportionately 
slightly larger. The diagrams were not made upon a definite scale. The 
numbering of the drawings is continuous with the preceding parts. 
Abbreviations : e. epidermis. 1111. — nutritive zone. 

ep. — epidermal zone. f. v. b. — fibro-vascular bundles, 

pa. — parenchyma zone. 1. c. — larval chambers, 

p. — protective zone. sc. — sclerenchyma. 

FLOWER AND FRUIT GAIvLS. 
70. Section of leaf of Euphorbia corollata. 

71a. Diagram of section of Phytoptus sp gall on leaf of E. corollata. 

71b. Section of 71a. 

72a. Section of lower part of ovary of E. corollata affected by Phytoptus sp . 



April, 1904.] Galls and Insects Producing Them. 135 

72b. Section of upper part of flower of E. corollata affected by Phytoptus sp . 

73a. Diagram of cross section of Cecidomyid bud gall on Solidago canadense. 

73b. Section of same. 

74a. Diagram of longitudinal section of Cecidomyid gall on Ratibida pinnata. 

74b. Diagram of longitudinal section of Cecidomyid gall on Ratibida pinnata. 

74c. Section of 74b. 

75a. Section of unaffected fruit of Primus virginiana. 

75b. Section of Cecidomyid gall developed in fruit of P. virginiana. 

ROOT GALL. 
76a. Section of young gall of Amphibolips radicola. 
76b. Section of mature gall of A. radicola. 

HISTOLOGY. 

77. Section of young gall of Phytoptus quadripes. 

78. Section of young gall of Phytoptus abnormis. 

79. Section of nutritive zone of young gall of Amphibolips inanis. 

80. Section of mature gall of A. inanis. 

81. Section of mature gall of Callirhyt is papillatus. (Nutritive, protective and part of 

parenchyma zones.) 

82. Section of mature gall of Dryophanta palustris. (Nutritive, protective and part of 

parenchyma zones.) 

83. Section of mature gall of Andricus petiolicola. 

SURFACE SECTIONS OF 

84. Dryophanta palustris. (Very young gall.) 
84b. Dryophanta palustris. (Mature gall.) 

Amphibolips inanis. 

86. Diastrophus siminis. 

87. Diastrophus potentilla. 
Pachypsylla c.-mamma. 
Colopha ulmicola. 

90. Phylloxera c.-globuli. 

91. Pemphigus p.-transversus. 

OVIPOSITORS OF 

92. Cecidomyia gleditsiae. 
93a. Nernatus salicis-ovum. 
93b. Nernatus salicis-ovum. 

94. Dryophanta palustris. 

95. Amphibolips radicola. 

96. Andricus cornigerus. 

97. Andricus seminator. 
98a. Rhodites radicum. 
98b. Rhodites radicum. 

MOUTHPARTS OF 

99. Colopha ulmicola. 

100a. Pachypsylla c.-mamma, with setae extended. 

100b. Pachypsylla c -mamma, with setae retracted. 

101. Cecidomyia gleditsiae. 

102. Cecidomyia pellex. 

103. Andricus petiolicola. 

104. Amphibolips inanis. 

105. Amphibolips confluentus. 
106a. Diastrophus siminis. 
106b. Diastrophus siminis. 

107. Callirhytis papillatus. 

108. Parasite from gall of C. papillatus. 

109. Holcaspis globulus, 
no. Nernatus pomum. 

in. Gelechia gallae-solidaginis. 



J 3 6 The Ohio Naturalist. 

Ohio Naturalist. 



[Vol. IV, No. 6, 



Plate IX , 




Cook on "Galls and Insects Producing Them. 



April, 1904.] Galls and Insects Producing Them 



Ohio Naturalist 




76' " V79~ 

Cook on "Galls and Insects Producing Them." 



i38 



The Ohio Naturalist. [Vol. IV, No. 6, 



Ohio Naturalist. 



Plate XI. 




Cook on " Galls and Insects Producing Them." 



April, 1904.] Galls and Insects Producing Them. 139 

Ohio Naturalist. p iate v// 




Cook on "Galls and Insects Producing Them. 



140 The Ohio Naturalist. [Vol. IV, No. 6, 

APPENDIX I. 



GALLS AND INSECTS PRODUCING THEM. 

Melville Thurston Cook. 
Part I. Morphology of Leaf Galls. 

I. GALLS OF THE APHIDIDAE. 

The gall of Pemphigus vagabundus Walsh (Fig. 112) is evi- 
dently formed as a result of the distortion of a large number of 
bud leaves. My specimens of these galls were mature, so I was 
unable to follow its development. Small fibro-vascular bundles 
were numerous and tannin was formed in great abundance. The 
structure was so modified that the leaf characters were lost ; the 
cells were uniform in character, but were slightly smaller near 
both the exterior and interior surfaces. 

The galls of Pemphigus rhois Fitch (Fig. 113) are large, blad- 
dery and evidently the pocketing of a single leaflet of the host 
plant, Rhus glabra or R. typhina. My specimens of these galls 
were fully mature, and I was therefore unable to follow the line 
of development. The leaf structure was modified into the char- 
acteristic Aphididae gall structure. Fibro-vascular bundles were 
numerous and near the inner surface of the gall. Opposite each 
bundle was a large cavity filled with some substance which I was 
unable to determine. 

2. GALLS OF CECIDOMYIDAE. 

The galls of Cecidomyia pellex O. S. (Figs. 114a, b) are formed 
by a thickening of the petiole, giving it the appearance of a long 
fleshy bean pod with a slit along the upper side. This gall shows 
three well defined zones ; an inner nutritive zone of small cells, a 
parenchyma zone of larger cells and the epidermal zone. The 
fibro-vascular bundles are numerous and are located between the 
nutritive and protective zones and arranged around the larval 
cavity and opening, the largest one just below the larval chamber 
and corresponding to the mid-rib of the leaflet. 

Cecidomyia impatientis O. S. (Fig, 115) is a fleshy gall occur- 
ring on the leaves of Impatiens fulva. Some of my specimens 
had the appearance of deformed flower buds, but upon this point 
I was unable to decide. This gall showed two well defined 
zones ; a zone of small cells lining the larval chamber and making 
up about one half the thickness of the gall, and an outer zone of 
large cells. Small fibro-vascular bundles were formed between 
the zones. 

The galls of Cecidomyia holotricha O. S. on Hicoria ovata 
(Figs. 116a, b, c) are small and very firm. My specimens were 



April, 1904.] Galls and Insects Producing Them. 141 

mature, but the cells lining the larval chamber were well supplied 
with protoplasm, and numerous short trichomes were developed 
from the dorsal surface and extended into the chamber. Tannin 
was very abundant. 

The gall of Cecidomyia tubicola O. S. on Hicoria ovata (Figs. 
1 17a, b, c) is very similar to C. holotricha, except that the amount 
of tannin is not so great. The upper wall of the gall is much 
thicker than either the side or lower wall. The point of attach- 
ment is not so large, but the gall is protected by a growth pro- 
ducing a cup-shaped cavity in which the gall is developed (Fig. 
117a). The inner layers of cells are very rich in protoplasm. 
The cells are elongated in the long axis of the gall and fibro- 
vascular bundles are more numerous than in C. holotricha, but 
are very small. The cup-shaped structure (117c) in which the 
gall is formed is composed of elongated cells. The palisade cells 
in that part of the leaf opposite the gall are unaffected. 

Cecidomyia viticola O. S. (Fig. 118) has the same general 
character as C, tubicola, but is much longer. 

Sciara ocellaris O. S. is one of the simplest of the Cecidomyidae 
galls. The larva does not penetrate the tissues of the leaf, but 
confines its attack to the outside, causing an indentation on one 
surface of the leaf and a corresponding elevation on the opposite 
surface (Fig. 119a) and also causing a very slight thickening. 
The structure (Fig, 1 19c) when compared with that of the normal 
leaf (Fig. 119b) shows the palisade transformed into ordinary 
mesophyll and the intercellular spaces entirely obliterated. It 
therefore corresponds in structure to the simple leaf-curl galls 
produced by some of the Aphididae (e. g., Schizoneura Ameri- 
cana Riley, Part 1, Fig. 12). 

3. GALLS OF THE CYNIPIDAE. 

My specimens of Rhodites bicolor Harris (Fig. 120) were well 
developed when collected. I was therefore unable to determine 
the early structural characters. The structure in these galls evi- 
dently does not show the four well defined zones so characteristic 
of this family. The inner cells are well supplied with nourish- 
ment for the large number of larvae. 

The galls of Amphibolips conflueutus Harris are very large and 
have a single larval chamber in th</. center. The nutritive and 
protective zones (Fig. 121a) can be distinguished, but are not so 
well defined as in the closely related species, A, inanis (Part I, 
Figs. 28a, b). The parenchyma and epidermal zones ( Fig, 121b) 
are well defined and the space in the parenchyma is filled with a 
cottony-like substance which upon close examination is composed 
of fibro-vascular bundles (as in A. inanis, Figs. 28a, b, and H. 
centricola, Figs. 27a, b, c) and of long, unicellular threads (Fig. 
121c), as in C. papillatus (Figs. 30a, b, c and 81). 



142 The Ohio Naturalist. [Vol. IV, No. 6, 

My specimens of Amphibolips illicifoliae Bassett were too far 
advanced to admit of sectioning, but a careful examination indi- 
cated that the zones were well defined and that the space in the 
parenchyma zone is bridged by means of fibro- vascular bundles 
as in A. inanis and H. centricola. 

The galls of Amphibolips primus Walsh (Fig. 122) are very 
firm and all the zones are well defined except the protective zone, 
which is entirely absent. The parenchyma zone is very thick 
and probably compensates for the lack of a protective zone. 
There are very few small fibro- vascular bundles. 

Galls of Amphibolips sculpta Bassett (Fig. 123) were more 
succulent than other specimens which I have examined. My 
specimens were mature, but the four zones were well defined. 
The nutritive zone was almost obliterated, due to the age of the 
gall. The protective zone was thin and the cell walls not very 
thick. The parenchyma zone was very thick and composed of 
large, succulent cells and was probably very important in furnish- 
ing nutriment to the larva. Near the outer surface were numer- 
ous small fibro-vascu'ar bundles. The epidermal zone was very 
prominent and composed of small cells. 

Andricus petiolicola Bassett is one of the firmest of the leaf 
galls. It is formed either on the petiole or mid-rib and is com- 
posed of very small, firm cells ( Fig. 124). The four zones are 
well defined, but the protective zone is very thin and the cell 
walls but very little thicker than in the neighboring cells The 
parenchyma zone is very thick, composed of very small cells with 
no intercellular spaces, but with many layers of long fibrous cells. 

The galls of Acraspis erinacei Walsh (Fig. 125) are very con- 
spicuous. The galls are always developed on the mid-rib of the 
leaf, but contain no fibro- vascular bundles. The nutritive zone 
is thick and very rich in protoplasm. The protective zone is 
also thick and gradually merges into the parenchyma zone, which 
is also thick. The epidermal zone is very irregular and is covered 
with numerous unicellular trichomes. 

The galls of Biorhiza forticornis Walsh are fig-shaped and the 
larval chamber instead of being suspended in the center of the 
gall, as is many others, is placed at the apex (Fig. 126a) and the 
space between the protective and parenchyma zones, or rather in 
the parenchyma zone, extends less than half way round the larval 
chamber. My specimens were mature and I was unable to make 
a careful study of the nutritive and protective zones. However, 
the nutritive zone appeared to be relatively thicker, while the 
protective zone was thin and merged gradually into the paren- 
chyma zone ( Fig. 126b). The parenchyma zone was thick and 
composed of large c^lls (Fig. 126c). Considerably more of this 
zone remained attached to the protective zone than is the case 
with most galls where this separation occurs. The cavity formed 



April, 1904.] Galls and Insects Producing Them. 143 

by the separation of the cells in this zone is bridged by numerous 
unicellular threads as in C. papillatus (Figs. 30a, b, c). In the 
outer part of the parenchyma zone, but near the cavity, are 
formed the fibro-vascular bundles. The epidermal zone is well 
defined and the trichomes on the surface are uni-cellular (Fig. 
126c). 

4. GALLS OF TENTHREDINIDAE. 

The galls of Nematus pomum Walsh were the only leaf galls of 
this family that I secured and they were mature. There was no 
indication of a zonal structure, but the cells were very uniform in 
size and structure throughout the entire gall ( Fig. 127). Many 
of the cells contained tannin and intercellular spaces were large 
and evenly distributed. 

Part II. Lateral Bud Galls. 

Mature specimens of Holcaspis globulus Fitch show the four 
well defined zones (Fig. 128). The inner nutritive zone is thick, 
composed of small cells and well supplied with nutriment for the 
larva. The protective zone is thin and composed of very small 
cells with thin walls. It gradually merges into the nutritive zone 
on the one side and the parenchyma zone on the other side. The 
parenchyma zone is very thick, the cell walls medium in size and 
the fibro-vascular bundles small and numerous. Further obser- 
vations upon this gall emphasize the statement previously made 
that it is the enlargement of an incipient stem. 

Further observations upon the gall of Andricus seminator 
Harris confirm the statement previously made that it is a com- 
pound gall produced by the insect depositing an egg in each 
element of the bud. 

Part III. Stem Galls. 

The gall of Diastrophus nebulosus O. S. (Fig. 129a, b) is a 
very large swelling on the canes of Rubus villosus and is about 
two or three inches in length. It contains a large number of 
larval chambers each containing a single larva (Fig. 129a). The 
four zones are especially well defined. The nutritive and protec- 
tive zones are composed of a few layers of cells while the paren- 
chyma zone is very thick, composed of smaller cells and more 
dense than the corresponding zone in most galls of this family. 

Andricus cornigerus O. S. (Fig. 130) produces one of the 
hardest of the stem galls. My specimens of this were gathered 
in the winter and were fully mature. The horn-like protuber- 
ance is a closed tube extending to near the center of the gall. 
This tube is composed of sclerenchyma tissue and evidently cor- 
responds to the protective zone. Near the base of the tube is a 
thin partition forming the larval chamber. When mature the 



144 The Ohio Naturalist. [Vol. IV, No. 6, 

insect destroys this partition, travels to the end of the tube which 
projects beyond the body of the gall, and there makes an opening 
through either the end or the side of the tube and thus makes its 
escape. Examination of young specimens would probably show 
the four zones as well defined as in Diastrophus nebulosus. 

Part IV. Development oe Galls. 

Examination of very young specimens of Andricus seminator 
Harris shows three well defined zones (Figs. 131a, b), the pro- 
tective zone being undeveloped. The fibro- vascular bundles were 
very numerous and distributed just beneath the epidermal zone. 
I have examined a large number of these galls of various ages 
and have been unable to find any trace of a protective zone. 
Tannin develops in the outer cells very early and probably helps 
to form a protection for the larva. 

PLATES xin-xv. 

112. Section of gall of Pemphigus vagabundus. 

113. Section of gall of Pemphigus rhois. 
114a. Diagram of gall of Cecidomyia pellex. 
114b. Section of gall of Cecidomyia pellex. 

115. Section of gall of Cecidomyia impatientis. 

116a. Diagram of the gall of Cecidomyia holotricha. 

116b. Section of the gall of Cecidomyia holotricha. 

116c. Section of the gall of Cecidomyia holotricha. 

117a. Diagram of the gall of Cecidomyia tubicola. 

117b. Section of the gall of Cecidomyia tubicola. 

117c. Section of the gall of Cecidomyia tubicola. 

118. Diagram of the gall of Cecidomyia viticola. 

119a. Diagram of the gall of Sciara ocellaris. 

119b. Section of normal leaf of Maple. 

119c. Section of gall of Sciara ocellaris. 

120. Section of gall of Rhodites bicolor. 

121a. Section of gall of Amphibolips confluentus. (Epidermal and parenchyma zones.) 

121b. Section of the gall of Amphibolips confluentus. Nutritive and protective zones. ) 

121C. Section of gall of Amphibolips confluentus. (Elongated cells in the cavity of the 
parenchyma zone.) 

122. Section of gall of Amphibolips piunus. 

123. Section of gall of Amphibolips sculpta. 

124. Section of gall of Andricus petiolicola. 

125. Section of gall of Acraspis erinacei. 
126a. Diagram of gall of Biorhiza forticornis. 

126b. Section of gall of Biorhiza forticornis. (Nutritive and protective zones.) 

126c. Section of the gall of Biorhiza forticornis. (Section of protective and epidermal 
zones. ) 

127. Section of the gall of Nematus pomum. 

128. Section of the gall of Holcaspis globulus. 
129a. Diagram of gall of Diastrophus nebulosus. 
129b. Section of gall of Diastrophus nebulosus 
130. Diagram of gall of Andricus cornigerus 

131a. Diagram of cross section of gall of Andricus seminator. 
131b. Section of young gall of Andricus seminator. 



April, 1904.] Galls and Insects Producing Them. 



M5 



Ohio Naturalist. 



Plate XIII. 




Cook on "Galls and Insects Producing Them. 



The Ohio Naturalist 



[Vol. IV, No. 6, 
Plate XIV. 




Cook on "Galls and Insects Producing Them." 



April, 1904.] Galls and Insects Producing Them. 



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